"The US 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 in a statement yesterday.
"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)," the White House said.
The United States is prepared to participate, the statement said. Obama spoke with Putin for 90 minutes, during which "President expressed his deep concern over Russia's clear violation of Ukrainian sovereignty and territorial integrity," the White House said.
Obama told 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."
"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 US 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.
The White House said Obama has directed his Administration to continue working urgently with international "partners" to provide support for the Ukrainian government, including urgent technical and "financial" assistance.