WASHINGTON -- U.S. President Barack Obama on Saturday called on Russian President Vladimir Putin to de-escalate the tense atmosphere in Ukraine by pulling his forces back to bases in the country's Crimean region and to refrain from interfering elsewhere in the former Soviet republic.

Obama personally delivered the message to Putin during a 90-minute telephone conversation, the White House said late Saturday.

But Obama's request appeared likely to go unheeded as the Kremlin said Putin, in turn, emphasized to Obama the existence of real threats to the life and health of Russian citizens living in Ukraine and that Russia has the right to protect its interests there.

Russian troops took over Crimea as the Russian parliament on Saturday granted Putin a green light to use the military to protect Russian interests in Ukraine.

Ukraine's newly installed government was powerless to react to the swarm of Russian troops.

"President Obama expressed his deep concern over Russia's clear violation of Ukrainian sovereignty and territorial integrity," the White House said in a statement that called the action "a breach of international law."

Hours earlier, Obama's national security team huddled at the White House to get updates on the situation and discuss policy options. Obama did not participate. Vice-President Joe Biden and Secretary of State John Kerry participated by videoconference.

"The United States condemns Russia's military intervention into Ukrainian territory," the White House said.

Obama also discussed the situation with French President Francois Hollande and Canadian Prime Minister Stephen Harper.

Defence Secretary Chuck Hagel spoke by telephone with his Russian counterpart and stressed that "without a change on the ground," Russia risks further instability in the region, isolation in the international community and an escalation that would threaten European and international security," the Pentagon said.

But a defiant-sounding statement issued by the Kremlin in Moscow said Putin stressed to Obama that the situation in Ukraine poses "real threats" to the life and health of Russian citizens and compatriots who live in Ukrainian territory.

"Vladimir Putin emphasized that, in the case of a further spread in violence in eastern regions (of Ukraine) and Crimea, Russia maintains the right to protect its interests and the Russian-speaking population that lives there," the Kremlin statement said.

Obama told Putin that the appropriate way for him to address concerns about the treatment of ethnic Russian and minority populations in Ukraine is to do so peacefully through direct contact with Ukraine's new government and through international observers.

Obama said the U.S. was prepared to help mediate such a dialogue.

Obama also made clear that Russia's continued violation of Ukraine's sovereignty and territorial integrity would have a negative effect on Russia's standing in the world.

Administration officials had said Obama might retaliate by cancelling a trip to Russia this summer for the Group of Eight international economic summit and could also cut off trade talks with Moscow. The White House said Saturday that the U.S. will suspend upcoming participation in "preparatory meetings" for the summit being planned for June in Sochi, site of the just completed Winter Olympics.

Political turmoil in Ukraine pushed President Viktor Yanukovych from office following massive pro-democracy protests after he rejected a partnership agreement with the European Union in favour of deepening his country's historical ties with Moscow.

Yanukovych held a news conference in southern Russia Friday in which he said he was not asking Moscow for military assistance and called military action "unacceptable." Yanukovych, who still considers himself Ukraine's president, also vowed to "keep fighting for the future of Ukraine" and blamed the U.S. and the West for encouraging the rebellion that forced him to flee last weekend.

The EU agreement Yanukovych rejected would have paved the way for Ukraine's greater integration with the West, including potential affiliation with NATO. Russia strongly objects to NATO membership for former Warsaw Pact members and Soviet republics.