WASHINGTON (AP) — One by one, President Barack Obama's warnings to Russia are being brushed aside by President Vladimir Putin, who appears to only be speeding up efforts to formally stake his claim to Ukraine's Crimean Peninsula.

In the week since Obama first declared there would be "costs" if Putin pressed into Crimea, Russian forces have taken control of the region and a referendum has been scheduled to decide its future. Obama declared the March 16 vote a violation of international law, but in a region where ethnic Russians are the majority, the referendum seems likely to become another barrier to White House efforts to compel Putin to pull his forces from Crimea.

"The referendum vote is going to serve for Putin, in his mind, as the credibility and legitimacy of Russia's presence there," said Andrew Kuchins, the director of the Russia program at the Center for Strategic and International Studies.

If Crimea votes to join Russia, the referendum could also put Obama in the awkward position of opposing the outcome of a popular vote.

The White House has tried to match Russia's assertive posture by moving quickly to impose financial sanctions and travel bans on Russians and other opponents of Ukraine's new central government. U.S. officials have also urgently tried to rally the international community around the notion that Russia's military maneuvers in Crimea are illegal, even seeking support from China, Moscow's frequent ally against the West.

"I am confident that we are moving forward together, united in our determination to oppose actions that violate international law and to support the government and people of Ukraine," Obama said Thursday.

The European Union also announced Thursday that it was suspending talks with Putin's government on a wide-ranging economic agreement and on granting Russian citizens visa-free travel within the 28-nation bloc — a long-standing Russian objective.

The White House says it still believes a diplomatic solution to the dispute with Russia is possible. Obama spoke with Putin for more than an hour Thursday, outlining a potential resolution that would include Russia pulling its forces back in Crimea and direct talks between the Kremlin and Ukraine.

But the fast-moving developments in Crimea may mean that the ultimate question facing Obama is not what the U.S. can do to stop Russia from taking control of Crimea, but what kind of relationship Washington can have with Moscow should that occur.

White House advisers insist the U.S. could not go back to a business as usual approach with Russia if Moscow were to annex Crimea or recognize its independence. But that may be seen as an empty threat to the Kremlin after the U.S., as well as Europe, did just that in 2008 after Russia recognized the independence of Abkhazia and South Ossetia, two breakaway territories of Georgia. Russia also continues to keep military forces in both territories.

Privately, U.S. officials say Russia is running a similar playbook as it seeks to increase its influence in Crimea. And regional experts say Putin also appears to have a larger goal: influencing central government lawmakers in the Ukrainian capital of Kiev as they prepare for elections later this spring.

"It says to the Ukrainians, 'Don't mess with me or I'll slice off a finger,'" said Matthew Rojansky, a Russia analyst at the Woodrow Wilson International Center for Scholars.

The months-long political crisis in Ukraine bubbled over late last month when protesters in Kiev ousted pro-Russian President Viktor Yanukovych. Amid the chaos, thousands of Russian forces took control of Crimea, a strategically important outpost in the Black Sea where Moscow has a military base.

The outcome of the Crimea referendum is not guaranteed, but there are clear indications the region will choose to side with Russia. About 60 percent of Crimea's population already identifies itself as Russian. And Crimea's 100-seat parliament voted unanimously Thursday in favor of joining Russia.

The speaker of Russia's upper house of parliament said Friday that Crimea would be welcome as an equal part of Russia if the region votes to leave Ukraine. Crimea would be the first territory to officially join Russia since the breakup of the Soviet Union in 1991.

The referendum had been scheduled for March 30, but was pushed up two weeks. And while the original vote was only on whether Crimea should get enhanced local powers, the peninsula's residents will now also vote on whether to join Russia.

U.S. officials say they believe Putin was involved in orchestrating the referendum, though the Russian leader made no public statements about the planned vote. Earlier in the week, Putin said Russia had no intention of annexing Crimea, while insisting its population has the right to determine the region's status in a referendum.

U.S. officials say they also see an unlikely ally emerging in China, which has frequently sided with Russia at the United Nations Security Council in blocking Western actions. While China has not condemned Russia's actions outright, Beijing's ambassador to the U.N. this week said it supported "noninterference" and respects Ukraine's sovereignty and territorial integrity.

Susan Rice, Obama's national security adviser, spoke this week with Chinese State Councilor Yang Jiechi. The White House said the officials agreed on the need for a peaceful resolution to the dispute that "upholds Ukraine's sovereignty and territorial integrity."

It appears unlikely China would actually take punitive actions against Russia. U.S. officials say Beijing is largely acting out of self-interest and appears to view the developments in Crimea through the prism of a nation that also has ethnic minorities who live in border regions and identify more closely with neighboring countries.

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