Simferopol, Ukraine (CNN) -- Russian President Vladimir Putin defended the breakaway moves by the pro-Russian leaders of Crimea in conversations with two world leaders on Sunday, while the autonomous Ukrainian region was the scene of emotional demonstrations by both sides.
In separate phone calls with German Chancellor Angela Merkel and British Prime Minister David Cameron, Putin underlined "steps being taken by Crimea's legitimate authorities ... being based on international law on behalf of the interests of the population there," according to a Kremlin statement. Putin also said new Ukrainian authorities were doing nothing "to curb ultra-nationalist and radical forces committing outrages" in Kiev and other regions.
The three leaders spoke amid tensions on the Black Sea peninsula that have escalated since the Moscow-backed regional parliament voted to leave Ukraine for Russia and announced a March 16 referendum to give Crimeans an opportunity to vote on the idea.
Despite differences of opinion over what is happening on the ground, the Kremlin statement said there was consensus on the need to de-escalate tensions and normalize the situation.
Moscow has denounced the events that led to Ukraine President Viktor Yanukovych's ouster as an illegitimate coup and has refused to recognize the new Ukrainian authorities, putting the two countries on a collision course over control of Crimea, which has longstanding ties to Russia and has thousands of Russian troops stationed there.
Putin has said Russia has the right to protect Russians living in the former Soviet republic. Pro-Russian forces are now in de facto control of the region ahead of the referendum, which Kiev says is illegal.
Washington has warned Moscow that any moves to annex Crimea would close the door to diplomacy. On Saturday, U.S. President Barack Obama rounded up world leaders to demand Russia "de-escalate the situation."
Ukrainian interim Prime Minister Arseniy Yatsenyuk will fly to the United States this week to discuss the crisis in Crimea, as hostilities in the eastern European country's southern region intensify.
Yatsenyuk is expected to arrive in the United States on Wednesday, a spokeswoman from his press office told CNN. The White House confirmed the visit and said the two will talk about the Russian military incursion and economic support for Ukraine.
In Simferopol, Crimea's main city, pro- and anti-Russian groups held rival rallies Sunday. Hundreds of supporters of Russia clapped along to nostalgic Soviet-era songs being played in a public square.
In Kiev, thousands of people gathered at a rally for peace at the central Independence Square, cradle of the protests that last month ousted Yanukovych.
The crowd shouted slogans such as "Glory for Ukraine" and "Putin go away" as representatives of different religions prayed for a solution to the crisis.
Crimea, a self-governing peninsula in southern Ukraine with an ethnic Russian majority and strong cultural ties to Russia, has become the epicenter of a battle for influence among Moscow, Kiev and the West since Yanukovych was pushed out of office by protesters who were angered over his rebuff of a trade deal with the European Union in favor of one with Russia.
Tensions in Crimea have been mounting since pro-Russian forces' bloodless seizure of the Black Sea peninsula last week. Amid signs that the tense standoff is growing more volatile, Russian troops also stormed a Crimean border control point at Schelkino, near Kerch, early Saturday, seizing the armory and driving the officers' families from their living quarters, Ukraine's border service said.
Putin last week secured permission from his parliament to use military force to protect Russian citizens in Ukraine. The move came within days after Yanukovych's flight from the country. Yanukovych was ousted after three months of protests against a decision to spurn a free trade deal with the European Union and turn toward closer ties with Moscow.
A convoy of military vehicles, believed to be carrying Russian soldiers, traveled through Simferopol on Saturday, heading toward the border post at Armyansk, a spokesman for the Ukrainian Ministry of Defense told CNN. Armyansk is one of the main access points into Crimea from the rest of Ukraine.
Dozens of military observers from the Organization for Security and Co-operation in Europe have tried to cross into Crimea for three days but have been refused entry by armed men. They did not attempt a crossing on Sunday.
U.S. steps up pressure
On Saturday, Obama called British, French and Italian leaders and hosted a conference call with the presidents of Lithuania, Latvia and Estonia.
"All of the leaders agreed on the need for Russia to pull its military forces back to their bases, allow for the deployment of international observers and human rights monitors to the Crimean peninsula, " a White House statement said.
The leaders also agreed on "the formation of a contact group that could lead to direct dialogue between Ukraine and Russia to de-escalate the situation and restore Ukraine's sovereignty and territorial integrity."
They also rejected a proposed referendum in Crimea on whether it should rejoin Russia "as a violation of Ukraine's constitution," the White House said. "The leaders made clear that Russia's continued violation of international law will isolate it from the international community."
U.S. Secretary of State John Kerry phoned Russian Foreign Minister Sergey Lavrov on Saturday and issued a diplomatic ultimatum, according to a senior U.S. administration official.
The referendum on whether the Crimean Peninsula should join Russia has become the focus of the Ukraine crisis. Yatsenyuk has called it "an illegitimate decision."
"If there is an annexation of Crimea, if there is a referendum that moves Crimea from Ukraine to Russia, we won't recognize it, nor will most of the world," U.S. deputy national security adviser Tony Blinken said on CNN's "State of the Union" on Sunday.
"So I think you'd see, if there are further steps in the direction of annexing Crimea, a very strong, coordinated international response."