US President Barack Obama has told Russian President Vladimir Putin that his 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.
US 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 AFP news agency reported.
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."
Canada recalls ambassador
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.
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.