President Barack Obama told President Vladimir Putin on Saturday that Russia's dispatch of troops to Ukraine flouted international law and warned he was courting political isolation if the incursion continues.
Obama also spelled out the right of the Ukrainian people to chart their own destiny and symbolically began to line up the long-time Western alliance against Russia, calling the leaders of France and Canada.
Secretary of State John Kerry also hosted a joint conference call with six other foreign ministers from Europe and Canada as well as EU foreign policy chief Catherine Ashton and the Japanese envoy to the US "to coordinate on next steps."
Obama's 90-minute telephone call with Putin represented the kind of direct confrontation between the men who run the White House and the Kremlin rarely seen since the end of the Cold War.
The White House account of the call was unusually detailed and blunt, hinting at tense exchanges as fractures deepened in a relationship that has been deteriorating since Putin returned as president in 2012.
"President Obama expressed his deep concern over Russia's clear violation of Ukrainian sovereignty and territorial integrity," the White House said.
Obama told Putin his actions were a "breach of international law, including Russia's obligations under the UN Charter, and of its 1997 military basing agreement with Ukraine."
Kerry also warned in a later statement that Moscow was risking the peace and security not just of Ukraine, but also the wider region.
If Russia did not de-escalate tensions, it would have a "profound" effect on ties with the US, said Kerry, who is due to meet his Russian counterpart Sergei Lavrov on the sidelines of talks in Rome next week.
Asked about the tone of the call, a senior US official resorted to diplomatic parlance indicating an uncomfortable conversation, describing it as "what you'd expect: candid and direct."
Obama team mulls options
Obama's national security team met at the White House to mull options on Ukraine, a day after the president warned Putin's actions would incur "costs."
Those costs would entail an immediate halt from the US side to preparatory talks on the G8 summit in the Olympic resort of Sochi on the Black Sea in June, Obama told Putin.
The crisis deepened after Putin secured an endorsement by lawmakers to send troops to Ukraine.
Officials in Kiev had earlier said Russia had already dispatched 30 armored personnel carriers and 6,000 additional troops into Crimea to help pro-Kremlin militia gain broader independence from the new pro-EU leaders in Kiev.
Obama called on Putin to pull his troops back to Russian barracks in the Crimean peninsula.
But in a sign his appeal fell on deaf ears, a Russian readout of the call hinted at an expansion of the operation, as Putin reserved the right to protect Russian interests in eastern Ukraine.
Obama suggested international observers appointed by the United Nations Security Council and the Organization for Security and Co-operation in Europe should be dispatched to safeguard ethnic Russians in Ukraine.
He also stated strong support for the Kiev government and pledged to work with bodies like the International Monetary Fund, the OSCE and NATO to mitigate its deepening economic crisis.
The US president called French President Francois Hollande and Canadian Prime Minister Stephen Harper, leaders of Atlantic nations along with Britain that formed the backbone of post-war Western resistance to the Soviet Union.
Condemning Russia's moves "in the strongest terms," Harper recalled his ambassador to Moscow and warned he may join Washington in snubbing June's G8 summit in Russia.
And Washington upped the diplomatic offensive at the United Nations, with US ambassador Samantha Power branding Russia's parliamentary approval "as dangerous as it is destabilizing."
"The message is pull back your forces. Let us engage in political dialogue, engage with the Ukrainian government which is reaching out to you for that dialogue," Power said.
But meaningful action on the crisis at the UN seems unlikely, given Russia's veto power as a permanent Security Council member.
Before Putin and Obama connected, US Defense Secretary Chuck Hagel called his Russian counterpart Sergei Shoigu.
A US defense official said there had been "no change" to Washington's defense posture in the European region.
Washington appears to have limited options to change Putin's calculations.
In addition to snubbing the G8 summit, it could cut off economic and trade cooperation that Moscow wants to deepen, or impose sanctions on Russian finance institutions or key officials.
Obama could order a show of military support for US allies in eastern Europe through NATO, but wants to avoid a Cold War-style chess match with Moscow.
He also needs Russian support for several key foreign policy priorities including nuclear talks with Iran and destroying Syria's chemical arms.
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