Mr Obama said he would seek to make sure key European leaders shared his view that Russia had failed to live up to the terms of a Ukraine peace accord in Geneva earlier this month, under which Russia, the United States, Ukraine and the European Union agreed to work to disarm illegal groups.
The Ukrainian government launched further military operations against some of the pro-Russian supporters who have seized government buildings across eastern Ukraine, having killed up to five rebels on Thursday.
Russia's foreign minister, Sergei Lavrov, accused authorities in Kiev of waging "war on their own people".
"This is a bloody crime, and those who pushed the army to do that will pay, I am sure, and will face justice," Mr Lavrov said.
Russia is also paying for the dispute, with heavy capital flight prompting credit rating agency Standard & Poor's to cut the country's ratings. That in turn forced Russia's central bank to raise its key interest rate to reverse a drop in the rouble.
Mr Lavrov said Moscow was committed to implementing the Geneva agreement, but accused Washington of distorting it with "one-sided demands".
However, Russia's Defence Ministry said it was ready for "unbiased and constructive" talks with the United States to stabilise the situation.
Mr Obama, who accuses Moscow of sending agents to co-ordinate the unrest in the east, as it did before seizing Ukraine's Crimea region in February, is planning to call allies in Europe later in the day.
"The window to change course is closing," US Secretary of State John Kerry warned Russia, citing Mr Obama's earlier comments that Washington was ready to impose new sanctions, on top of those imposed after Crimea was annexed.
Mr Kerry said Russia was using propaganda to hide what it was trying to do in eastern Ukraine, which it believes is to destabilise the region and undermine next month's Ukrainian presidential elections - and decried its "threatening movement" of troops up to Ukraine's border.
"If Russia continues in this direction, it will not just be a grave mistake, it will be an expensive mistake," Mr Kerry said.
Russian President Vladimir Putin has scoffed at the sanctions so far imposed, which have been limited to travel bans and overseas assets freezes on individuals.
Ukraine's Prime Minister Arseny Yatseniuk said Russia wanted to start the Third World War by occupying the country "militarily and politically" and creating a conflict that would spread to the rest of Europe.
Ukraine said Russian troops conducting exercises had approached to within just over half a mile of its border and would treat any incursion as an invasion.
Ukrainian special forces launched a second phase of their operation in the east of the country by mounting a full blockade of the rebel-held city of Slaviansk, an official on the presidential staff said.