"The leaders expressed their grave concern over Russia's clear violation of Ukrainian sovereignty and territorial integrity, which is a breach of international law and a threat to international peace and security," the White House said in a readout of the talks. "The leaders stressed that dialogue between Ukraine and Russia should start immediately, with international facilitation as appropriate."

Obama is sending his top envoy John Kerry to Kiev, Ukraine's capital, on Tuesday.

Russian President Vladimir Putin on Saturday asked for and was granted authorization to use military force to protect Russian interests in Ukraine.

In a telephone conversation Sunday with Obama, Putin said his country retained the right to protect its interests and Russian speakers living in Ukraine if violence spread in eastern Ukrainian regions and Crimea in the south.

Citing "real threats" to life and health of Russian citizens and Russian-speakers living in Ukraine, Putin attributed Russia's possible military move to "provocative and criminal actions by ultra-nationalistic elements that are actually encouraged by current authorities in Kiev," the Interfax news agency said.

In a conversation with U.N. Secretary-General Ban Ki-moon, Putin said Russia could not stay aside if violent actions were taken against Russian-speakers in eastern Ukrainian regions and Crimea. Russia and Ukraine have deep cultural, historical and economic connections.

Russia would take necessary measures according to international law, he added. But both sides agreed that further escalation of the crisis in Ukraine should be averted.

The move had sparked an immediate outcry from Western countries, which have been jockeying for influence in the former Soviet republic with Russia.

In their phone talks, Obama and the other leaders also pledged to work together on a package of multilateral and bilateral financial aid to help Ukraine as it pursues reforms to stabilize its economy, the White House said.

Also on Sunday, Russian Prime Minister Dmitry Medvedev said on Facebook said Russia is ready to develop mutually beneficial and effective relations with Ukraine. "But Ukraine for us is not a group of people who, pouring blood on the Maidan (Kiev's main square), seized power in violation of the constitution and other state laws," he added.

Medvedev also said ousted President Viktor Yanukovych remained the legitimate head of state according the constitution, adding: "If he is guilty before Ukraine -- hold an impeachment procedure ... and try him," he added.

Xinhuanet