Kiev, Ukraine (CNN) -- Ukraine's new leaders accused neighboring Russia of declaring war, as Kiev mobilized troops and called up military reservists in a rapidly escalating crisis that has raised fears of a conflict.
Amid signs of Russian military intervention in Ukraine's Crimean peninsula, Russian generals led their troops to three bases in the region Sunday, demanding Ukrainian forces surrender and hand over their weapons, Vladislav Seleznyov, spokesman for the Crimean Media Center of the Ukrainian Defense Ministry, told CNN.
By late Sunday, Russian forces had "complete operational control of the Crimean Peninsula," a senior U.S. administration official said. The United States estimates there are 6,000 Russian ground and naval forces in the region, the official said.
"There is no question that they are in an occupation position -- flying in reinforcements and settling in," another senior administration official said.
Speaking by phone, Seleznyov said Russian troops had blocked access to bases but added, "There is no open confrontation between Russian and Ukrainian military forces in Crimea" and said Ukrainian troops continue to protect and serve Ukraine.
"This is a red alert. This is not a threat. This is actually a declaration of war to my country," Ukrainian interim Prime Minister Arseniy Yatsenyuk said.
Speaking in a televised address from the parliament building in the capital, Kiev, he called on Russian President Vladimir Putin to "pull back his military and stick to the international obligations."
"We are on the brink of the disaster."
A sense of escalating crisis in Crimea -- an autonomous region of eastern Ukraine with strong loyalty to neighboring Russia -- swirled, with U.S. Secretary of State John Kerry condemning what he called Russia's "incredible act of aggression."
Speaking on the CBS program "Face The Nation," Kerry -- who is set to arrive in Kiev on Tuesday -- said several foreign powers are looking at economic consequences if Russia does not withdraw its forces.
"All of them, every single one of them are prepared to go to the hilt in order to isolate Russia with respect to this invasion," he said. "They're prepared to put sanctions in place, they're prepared to isolate Russia economically."
But Ukraine's ambassador to the United Nations said his country needs more than diplomatic assistance.
"We are to demonstrate that we have our own capacity to protect ourselves ... and we are preparing to defend ourselves," Yuriy Sergeyev said on CNN's "State of the Union." "And nationally, if aggravation is going in that way, when the Russian troops ... are enlarging their quantity with every coming hour ... we will ask for military support and other kinds of support."
In Brussels, Belgium, NATO ambassadors held an emergency meeting on Ukraine.
"What Russia is doing now in Ukraine violates the principles of the U.N. charter," NATO Secretary General Anders Fogh Rasmussen told reporters. He later added that Russia's actions constituted a violation of international law.
He called upon Russia to honor its international commitments, to send it military forces back to Russian bases, and to refrain from any further interference in Ukraine.
Rasmussen also urged both sides to reach a peaceful resolution through diplomatic talks and suggested that international observers from the United Nations should be sent to Ukraine.
German Chancellor Angela Merkel's office said Putin had accepted a proposal to establish a "fact-finding mission" to Ukraine, possibly under the leadership of the Organization for Security and Co-operation in Europe, and to start a political dialogue.
Lean to the West, or to Russia?
Ukraine, a nation of 45 million people sandwiched between Europe and Russia's southwestern border, has been plunged into chaos since the ouster of President Viktor Yanukovych on February 22 following bloody street protests that left dozens dead and hundreds wounded.
Anti-government protests started in late November when Yanukovych spurned a deal with the EU, favoring closer ties with Moscow instead.
Ukraine has faced a deepening split, with those in the west generally supporting the interim government and its European Union tilt, while many in the east prefer a Ukraine where Russia casts a long shadow.
Nowhere is that feeling more intense than in Crimea, the last big bastion of opposition to the new political leadership. Ukraine suspects Russia of fomenting tension in the autonomous region that might escalate into a bid for separation by its Russian majority.
Ukrainian leaders and commentators have compared events in Crimea to what happened in Georgia in 2008. Then, cross-border tensions with Russia exploded into a five-day conflict that saw Russian tanks and troops pour into the breakaway territories of South Ossetia and Abkhazia, as well as Georgian cities. Russia and Georgia each blamed the other for starting the conflict.
At Ukraine's Perevalnoye base, some 20 kilometers (12 miles) from Crimea's regional capital of Simferopol, a CNN team saw more than 100 troops -- not Ukrainian and dressed in green with no identifiable insignia -- deployed around its perimeter, as well as a dozen or so vehicles. Some 15 Ukrainian soldiers were on guard while civilians, both pro-Russia and pro-Ukraine protesters, stood on each side of the road.
A 66-year-old man named Nikolai Petukhov marched up to the entrance of the military facility carrying a Russian flag. He told CNN that he hoped Putin would facilitate democratic elections in Ukraine.
When asked whether he thinks Crimea should be part of Russia or Ukraine, he said, "If you look at it logically, it should be part of Russia."
It is not an unpopular feeling there, as 58% of the 2,033,000 residents of Crimea identified themselves as Russian in a 2001 census.
In Simferopol, men dressed in both civilian clothes and camouflage gear and wearing red armbands were seen on the streets.
By Sunday night, electricity had been cut off at the headquarters of the Ukrainian Navy in Crimea, and officials feared there could soon be an attack, Seleznyov said.
CNN has not independently verified that claim, and Russian officials could not be immediately reached to respond.
Word of the power outage came hours after the newly named head of Ukraine's navy disavowed Ukraine's new leaders and declared his loyalty to the pro-Russian, autonomous Crimea government.
Rear Adm. Denis Berezovsky, who was appointed Saturday by interim Ukrainian President Oleksandr Turchynov, said from Sevastopol on the Black Sea that he will not submit to any orders from Kiev.
He was quickly suspended and replaced by another rear admiral, the Defense Ministry in Kiev said in a written statement.
These scenes come one day after Putin obtained permission from his parliament to use military force to protect Russian citizens in Ukraine, spurning Western pleas not to intervene.
Putin cited in his request a threat posed to Russian citizens and military personnel based in southern Crimea.
Ukrainian officials have vehemently denied Putin's claim.
At a Ukrainian parliamentary meeting Sunday, acting Defense Minister Ihor Tenyuh said Ukraine does not have the military force to resist Russia, according to two parliamentary members present at the session. Tenyuh called for talks to resolve the crisis with Russia, they said.
The Ukrainian National Security Council has ordered the mobilization of troops, and the Defense Ministry was calling for reservists to register to be on standby if needed, a senior Ukrainian official, Andriy Parubiy, said.
In Kiev, thousands of people rallied in the central Independence Square, the cradle of Ukraine's three-month anti-government protests that led to Yanukovych's ouster. The crowd held up signs reading "Crimea, we are with you" and "Putin, hands off Ukraine."
In Moscow, about 50 protesters were detained outside a Defense Ministry building, a Moscow police spokesman said.
According to a tweet from the official Russian government account Sunday, Russian Prime Minister Dmitry Medvedev discussed the crisis in Ukraine in a telephone call with Yatsenyuk. According to a second tweet, Medvedev said Russia is interested in maintaining stable and friendly relations with Ukraine but reserves the right to protect the legitimate interests of its citizens and military personnel stationed in Crimea.
Western governments worried
The crisis set off alarm bells in the West.
In discussions over the weekend with Putin, U.S. President Barack Obama "made clear that Russia's continued violation of Ukraine's sovereignty and territorial integrity would negatively impact Russia's standing in the international community," according to a statement released by the White House.
According to the Kremlin, Putin told Obama that Russia reserves the right to defend its interests in the Crimea region and the Russian-speaking people who live there.
Obama met Sunday with his national security team and was scheduled to call U.S. allies afterward, White House spokesman Josh Earnest said.
UK Prime Minister David Cameron also planned to talk with Lithuanian President Dalia Grybauskaite and Polish Prime Minister Donald Tusk.
Britain's Foreign Minister William Hague on Sunday arrived in Kiev where he will meet with Ukraine leaders.
Canada recalled its ambassador to Moscow, while the United States and Britain announced they will suspend participation in preparatory meetings this week ahead of the G8 summit that will bring world leaders together in June in Sochi, Russia. France said it made the same decision.