By Justin Sink - 06/23/14 02:46 PM EDT
President Obama asked Russian President Vladimir Putin to "promote greater peace" in Ukraine as Kiev and pro-Russian separatists entered into a cease-fire agreement leaders hope can help resolve the months-long conflict there.
"Though we believe a diplomatic solution is still possible, Russia will face additional costs if we do not see concrete actions to de-escalate the situation."
In a statement, the Kremlin said Putin stressed to Obama "that ending military actions and starting direct talks between the conflicting sides would have priority significance for normalizing the situation."
"The leaders also noted the importance of urgently resolving humanitarian issues, including immediate assistance to the people who have been affected," the Kremlin said.
Earlier Monday, rebels in Ukraine's eastern region said they would honor a cease-fire offered by newly inaugurated Ukrainian President Petro Poroshenko. A representative for Poroshenko also met with rebel leaders on Monday, The Wall Street Journal reported.
Those were promising signs for a thaw between the two sides, and a step toward reconciliation after the U.S. announced last week that Russia had sent tanks and rocket launchers across the border to aid separatists.
A U.S. official said Friday that the U.S. was engaging in "active conversation" with Europe on sanctions that would narrowly target Russia's technology, defense and financial sectors.
The administration also imposed economic sanctions on seven Ukrainian separatist leaders last week.
According to the Kremlin, the leaders also briefly discussed the situation in Syria. Earlier Monday, the last of Syria's declared chemical weapons left the country, completing an important stage of the Moscow-brokered deal between the Syrian government and the U.S.