WASHINGTON (AFP) - United States (US) President Barack Obama told President Vladimir Putin in a 90-minute phone call on Saturday that Russia had violated international law with its incursion into Ukraine and warned of US and allied reprisals.
The White House said Mr Obama called on Mr Putin to pull Russian troops back to their bases in Crimea and warned that continued violation of international law by Moscow would lead to further "political and economic isolation".
The call came as Mr Obama's national security team met to draw up US responses to the escalating crisis, after Russia's parliament gave Mr Putin the formal go ahead to send forces to Ukraine and officials in Kiev said 6,000 forces had already been sent to Crimea.
"President Obama expressed his deep concern over Russia's clear violation of Ukrainian sovereignty and territorial integrity," the White House said in a detailed statement about the call.
Mr Obama told Mr Putin that 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".
"The United States calls on Russia to de-escalate tensions by withdrawing its forces back to bases in Crimea and to refrain from any interference elsewhere in Ukraine," the statement said.
Mr Obama also warned Mr Putin that the people of Ukraine had the right to determine their own future.
"Russia's continued violation of international law will lead to greater political and economic isolation," the statement added.
Mr Obama informed Mr Putin, with whom he has had tense public interactions, that he would order US diplomats to stop preparatory meetings for the G8 summit in Sochi, Russia in June.
On Friday, US officials said Obama and other European leaders were unlikely to show up at the summit in the showcase Black Sea report if the Ukraine crisis was not resolved.
Mr Obama did use the call to recognize that there were deep cultural ties between Ukraine and Russia and that there was a need to protect ethnic Russians in the former Soviet republic.
But he argued that the proper way to do that was through a dialogue with the government in Kiev and with international observers on the ground appointed by the United Nations Security Council and the Organisation for Security and Co-operation in Europe (OSCE).
Mr Obama also vowed to keep up consultations with the interim government appointed in Ukraine after the ouster pro-Russian president Viktor Yanucovych and bodies like the International Monetary Fund to support Kiev and to mitigate its deepening economic crisis.