President Barack Obama has warned his Russian counterpart Vladimir Putin against violation of Ukrainian sovereignty and asked him to pull the forces back to their bases in Crimea, saying Moscow’s refusal would lead to suspension of the U.S. participation in the upcoming G-8 summit.

In a 90-minute phone call, Mr. Obama expressed “deep concern” over Russia’s “violation of Ukrainian sovereignty and territorial integrity” and asked Mr. Putin to withdraw forces to bases in Crimea or face political and economic isolation.

“The U.S. 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 White House said after the phone call between the two leaders on Saturday.

“Obama told Putin that, if Russia has concerns about the treatment of ethnic Russian and minority populations in Ukraine, the appropriate way to address them is peacefully through direct engagement with the government of Ukraine and through the dispatch of international observers under the auspices of the United Nations Security Council or the Organisation for Security and Cooperation in Europe (OSCE),” it said in a statement, adding the U.S. is ready to participate.

He told Mr. Putin that his actions were a “breach of international law, including Russia’s obligations under the U.N. Charter, and of its 1997 military basing agreement with Ukraine.”

Mr. Putin told Mr. Obama there was a “real threat weighing on the lives and the health of Russian citizens” in Ukraine.

The Russian president accused the new government in Kiev of supporting “criminal actions by ultranationalists”, the Kremlin said in statement.

It said the conversation was instigated by the U.S. side and focused on the “extraordinary situation in Kiev“.

The call came after Mr. Obama’s national security team met to weigh policy options as Washington tried to keep pace with the fast escalating crisis in Ukraine and Crimea.

“We have consistently said that we recognise Russia’s deep historic and cultural ties to Ukraine and the need to protect the rights of ethnic Russian and minority populations within Ukraine,” the White House said.

“The Ukrainian government has made clear its commitment to protect the rights of all Ukrainians and to abide by Ukraine’s international commitments, and we will continue to urge them to do so,” it said.

“In the coming hours and days, the United States will urgently consult with allies and partners in the UN Security Council, the North Atlantic Council, the Organisation for Security and Cooperation in Europe, and with the signatories of the Budapest Memorandum,” the White House said.

“The U.S. will suspend upcoming participation in preparatory meetings for the G-8. Going forward, Russia’s continued violation of international law will lead to greater political and economic isolation,” it said.

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