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The U.S. is preparing to pull out of the next summit of industrialized nations — scheduled in Russia — in protest of Russian military moves in Ukraine, the White House said Saturday after President Barack Obama and Russian President Vladimir Putin spoke directly by phone.

As tension in Ukraine threatened to boil over, Obama told Putin in the 90-minute call that he was deeply concerned over Russia's "clear violation of Ukrainian sovereignty and territorial integrity," the White House said in a statement, adding that the president warned Putin that Russia's "standing in the international community" was at stake.

The statement said the U.S. was suspending "upcoming participation in preparatory meetings" for the G8 summit scheduled for June in Sochi, Russia.

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"Going forward, Russia's continued violation of international law will lead to greater political and economic isolation," it promised.

The Kremlin press office, which said Obama initiated the call, said Putin stressed what he called "the provocative and criminal actions on the part of ultranationalists," who he said were supported "by the current authorities in Kiev."

Citing "a real threat to the lives and health of Russian citizens," Putin told Obama that "Russia retains the right to protect its interests and the Russian-speaking population," the Kremlin said.

But Obama responded that the appropriate way to address those issues is peacefully "through direct engagement with the government of Ukraine," the U.N. Security Council or the Organization for Security and Cooperation in Europe, the White House said.

Secretary of State John Kerry echoed the White House warning, saying in a separate statement: "Unless immediate and concrete steps are taken by Russia to deescalate tensions, the effect on U.S.-Russian relations and on Russia's international standing will be profound."