(CNN) -- A day after taking over Mosul, Iraq's second-largest city, militants gained nearly complete control of the northern city of Tikrit, witnesses in the city and police officials in neighboring Samarra told CNN.
Heavy fighting erupted inside Tikrit -- the hometown of former Iraqi leader Saddam Hussein -- as the military tried to regain control of the city, the sources and a police official in Baghdad said.
According to the witnesses in Tikrit and the Samarra police officials, two police stations in Tikrit had been set on fire and a military base taken over by militants, who are believed to be from the Islamic State of Iraq and Syria, an al Qaeda splinter group also known as ISIS and ISIL.
The governor of Salaheddin province, of which Tikrit is the capital, was missing, according to the Tikrit and Samarra sources.
Suspected ISIS militants raided the Turkish Embassy in Mosul on Wednesday, capturing 48 people, including diplomats. They also seized parts of Baiji, the site of Iraq's largest oil refinery, police officials in Tikrit told CNN earlier.
Meanwhile, explosions struck three Shiite areas in Baghdad on Wednesday, killing 25 people and injuring 56, police officials told CNN. The deadliest attack was in Sadr City, where a car bomb exploded near a funeral tent, killing 15 people and wounding 34, police said.
And in Sefwan, a city on the Kuwait border about 310 miles (500 kilometers) south of Baghdad, a car bomb exploded near an outdoor market, killing at least three people and injuring 12 others, police in Basra said.
The violence comes on the heels of a sudden and danger-fraught exodus from the fighting in Mosul, which fell to militants Tuesday.
More than 500,000 people have fled the fighting there, the International Organization for Migration said Wednesday.
The group, which has been monitoring the plight of those caught in the onslaught, said the violence had resulted in "a high number of casualties among civilians."
The northern city's four main hospitals are inaccessible because of fighting, and some mosques have been converted for use as clinics, the IOM said.
Those fleeing the fighting, in vehicles or on foot, some bringing only what they can carry in plastic bags, are heading to the city's east or seeking sanctuary elsewhere in Nineveh province or in Iraq's Kurdish region.
The rush led to bottlenecks at checkpoints Tuesday as people tried to reach safety in nearby Erbil.
Despite its size -- the predominantly Sunni city has a population of about 1.6 million -- Mosul's collapse was swift. After weekend clashes, hundreds of radical Islamist fighters swarmed through the west of the city overnight Monday to Tuesday.
American-trained Iraqi government forces fled in the face of the onslaught by the suspected ISIS fighters. The militants now control most, if not all, of the city.
The heavily armed radicals overran police stations, freed more than 1,000 prisoners from the city jail and captured the international airport.
ISIS issued a statement promising to continue its "series of blessed battles" in Nineveh province, which it said was designed to "open up the province completely and cleanse it of apostates."
Iraq's parliamentary speaker was scathing, saying the Iraqi forces "abandoned their weapons and the commanders fled, leaving behind weapons, armored vehicles. Their positions were easy prey for terrorists."
Prime Minister Nuri al-Maliki ordered that all military leaders who fled be court-martialed.
Later, Iraq's Defense Ministry said the air force had struck a group of ISIS militants along a highway leading south toward Samara, killing all of them. The ministry also said it would push back the militants.
"This is not the end, we are very confident that we will be able to correct the path and to overcome mistakes," the ministry said in a statement posted to its website.
The Interior Ministry said that military commanders have started deploying fighters from local Shiite militias on the western outskirts of al-Nasiriya to protect that city from militant attacks.
Forces from the Kurdistan regional government took up positions in southwest Kirkuk after militants took over several villages and districts north and west of the city and the Iraqi army withdrew, police officials there told CNN.
The Kurdish regional prime minister -- whose ethnic Kurdish forces reach the eastern outskirts of Mosul, capital of Nineveh province -- blamed Iraq's leadership for the city's collapse.
"Over the last two days, we tried extremely hard to establish cooperation with the Iraqi Security Forces in order to protect the city of Mosul. Tragically, Baghdad adopted a position which has prevented the establishment of this cooperation," Nechirvan Barzani said in a statement Tuesday.
Turkish consulate targeted
Turkish special forces members, consulate workers and three children were among those detained and taken to the ISIS headquarters following a raid on the Turkish Consulate in Mosul on Wednesday morning, Turkish officials told CNN.
"The condition of the Turkish citizens is fine, developments are being monitored," the officials said. "It is a quite fragile situation, but hopefully we will resolve it today or in a couple days."
Turkey's Foreign Ministry said Tuesday that militants abducted 28 Turkish truck drivers hauling fuel from Turkey to a plant in Mosul.
Foreign Minister Ahmet Davutoglu said consulate staff had been urged to leave this week, but the decision to evacuate was left up to individuals.
"We were told that it would be more risky for our 48 people to go outside than to stay inside," Davutoglu said, speaking on Turkish television.
He warned militants not to harm the captives.
"If any harm is done to any of our citizens, it will not go unanswered," he said. "No one should test Turkey."
Oil town under attack
Meanwhile, suspected ISIS militants on Wednesday seized parts of Baiji, a small Iraqi town in Salaheddin province about 200 kilometers (125 miles) north of the capital, Baghdad, police officials in Tikrit told CNN.
The Baiji oil refinery -- Iraq's largest -- is still under the control of Iraqi security forces, officials said.
The fact that ISIS forces are trying to take the town will worry the oil industry in Iraq but also suggests a wider strategic aim.
Baiji sits on the main highway north from Baghdad to Mosul -- a road that passes through rural areas in which ISIS has a lot of influence.
For the government to reinforce its troops in Mosul, it needs to drive them through Baiji. If ISIS controls the town, or at least can pour firepower on the highway, it will make it much harder for the government to give that support.
The move into Salaheddin province shows how close the major fighting is getting to Baghdad.
In the Baghdad area, a car bomb exploded at a funeral in the suburb of Sadr City on Wednesday, killing three and wounding eight, according to police officials in Baghdad.
Discontent feeds violence
In his weekly address to the nation Wednesday, al-Maliki described the assault on Mosul as a "conspiracy" to destabilize the country and called on Iraqis to "stand as one united front."
He also praised the people of Nineveh province for volunteering to take up arms against ISIS and promised to "cleanse Nineveh from these terrorists."
A day earlier, the Prime Minister asked for parliament to declare a state of emergency and for volunteers to pick up guns and bolster the army. He also requested help from the international community.
Shiite cleric Muqtada al-Sadr said in a statement Wednesday that he is ready to form a "peace brigade" to work in coordination with the Iraqi government "to defend the holy places" of Muslims and Christians.
But this brigade probably would be viewed by many as a resurgence of al-Sadr's Mahdi Army, the powerful Shia militia formerly active in Sadr City and officially disbanded at the end of 2008.
Its formation could risk worsening the country's underlying problem -- festering sectarian division.
The country's minority Sunni population, which prospered under Hussein, feels shut out by al-Maliki's Shia majority-dominated government.
It's a discontent that feeds growing sectarian tensions that find expression in multiple daily car bombings and suicide attacks.
On Saturday, there were six roadside bombings in Baghdad alone, in which 33 people were reported killed and 72 wounded.
The devastating ISIS advance, which had been building for some time, is proving an object lesson of much that is wrong in Iraq and the region -- with a festering civil war over the border in Syria adding fuel to the growing sectarian tensions at home.
ISIS is exploiting this to expand its influence, from cities like Falluja and parts of Ramadi that it wrested from the government in Anbar early this year, and from Syrian towns like Raqqa it controls over the border.
A U.S. counterterrorism official told CNN that ISIS had been active in Nineveh province "for a long time and clearly sensed that Mosul was vulnerable now after engaging in sporadic attacks earlier this year.
"Strategically, the group looks at Syria and Iraq as one interchangeable battlefield, and its ability to shift resources and personnel across the border has measurably strengthened its position in both theaters."
However, the official said, despite the territorial advances it has made in Sunni-dominated Anbar and Nineveh provinces, ISIS still has "significant weaknesses."
"It has shown little ability to govern effectively, is generally unpopular and has no sway outside the Sunni community in either Iraq or Syria."
Too radical for al Qaeda
The more the Sunnis feel they are being abandoned by their Shia-dominated government, the harder any political rapprochement, and therefore peace, will be.
ISIS is exploiting this weakness. It is considered too radical even for al Qaeda and in the past months has withstood and emerged from a jihadist backlash from within the ranks of its erstwhile radical Islamist allies in Syria's civil war.
That it is capable of fighting the forces of Syrian President Bashar al-Assad on one hand, its fellow radicals on another and the Iraqi government on top of that -- where it is winning significant battles and scoring massive weapons hauls -- is an indication of the depth to which ISIS has established itself in the region.
According to the United Nations, last year was Iraq's most violent in five years, with more than 8,800 people killed, most of them civilians.
This year, almost half a million people have been displaced from their homes in central Anbar province.
ISIS grew out of al Qaeda in Iraq. In the west of Iraq, its militants were responsible for the deaths and maiming of many U.S. troops. In 2006, their commander -- the bloodthirsty Abu Musab al-Zarqawi -- was killed in a U.S. strike.
In the ensuing years, with American help, Iraqi tribal militias put the al Qaeda upstart on the defensive.
But when U.S. troops left, the extremist militants returned, found new leadership, went to Syria, grew stronger, and came back to Iraq, making military gains often off the backs of foreign fighters drawn to Syria's conflict.
They came to Syria's civil war better equipped and trained than most jihadists, with skills learned fighting in Iraq. They exploited their advantage, charting a course directed by a vision for a regional caliphate.
Mosul has not just helped fill their war chest, it has made them the single most dangerous destabilizing radical group in the region -- something the Iraqi government seems ill-equipped to deal with.