U.S. President Barack Obama was cautious on Thursday about an apparent breakthrough agreement with Russia over Ukraine.

Obama said the talks, aimed at ending tensions in Ukraine, are promising, but that the United States and its allies are prepared to impose sanctions on Russia if the situation fails to improve.

"There is the possibility, the prospect, that diplomacy may de-escalate the situation," he told reporters at the White House.

"The question now becomes will in fact they use the influence they've exerted in a disruptive way to restore some order so that Ukrainians can carry out an election and move forward with the decentralization reforms that they've proposed," he said. He added that that won't be clear for "several days."

"I don't think we can be sure of anything at this point ... I don't think given past performance that we can count on [Russia's follow-through] and we have to be prepared to respond to what appears to be efforts of interference by Russia."

The U.S. president said he had spoken with German Chancellor Angela Merkel earlier in the day, and planned later to talk to British Prime Minister David Cameron.