Disagreements among European Union nations on whether to impose new economic sanctions on Russia have held up punitive steps by the United States, said the sources on condition of anonymity, but Washington could also act on its own.

U.S. officials have grown increasingly impatient with what they describe as Russia's failure to live up to its commitments in an April 17 agreement reached in Geneva to try to de-escalate the crisis in Ukraine.

The United States is also frustrated at the reluctance of some European nations, notably Germany and Italy, to impose a new round of economic sanctions on Russia but it would much prefer to act in concert with the EU rather than on its own.

The sources said Obama was expected to speak in a conference call on Friday with British Prime Minister David Cameron, French President Francois Hollande, German Chancellor Angela Merkel and Italian Prime Minister Matteo Renzi to discuss the issue.

A senior administration official confirmed that the president intended to call European allies to discuss sanctions.

Obama is in Asia on a four-nation trip. Speaking in Tokyo on Thursday, he blamed Russia for failing to carry out the Geneva deal and said he was ready to impose new sanctions.

In a sign of growing U.S. concern about Ukraine, Secretary of State John Kerry issued what amounted to a warning to Russia not to invade. Russia has some 40,000 troops on its border with Ukraine, some of which staged military exercises on Thursday.

The United States has so far imposed three rounds of sanctions in connection with the unrest in Ukraine - two aimed at Russian targets and a third focused on Crimean individuals and a Crimean gas company.

The European Union is highly dependent on Russian gas deliveries, and the crisis over Ukraine has fanned concerns about future supply. Russia is also an important market for many EU exporters, notably in Germany.

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