President Barack Obama said Friday he will talk to European leaders later on Friday about Ukraine and may be ready to impose more sanctions if Russia escalates actions.
Obama, on a visit to the South Korean capital, wants to nudge the EU toward fresh sanctions against Russia over Ukraine, sources familiar with the matter said earlier.
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.
Obama will 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.
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.
A grave and expensive mistake
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.
"Following today's threatening movement of Russian troops right up to Ukraine's border, let me be clear: If Russia continues in this direction, it will not just be a grave mistake, it will be an expensive mistake," he said in a hastily arranged appearance at the State Department.
The United States accuses Russia of backing separatists in eastern Ukraine as part of a deliberate attempt to destabilize the region, undermine elections planned for next month, and gain greater influence over Kiev.
Russia seized and annexed the Crimean peninsula from Ukraine last month after President Vladimir Putin overturned decades of post-Cold War diplomacy by announcing the right to use military force in neighboring countries.
Under the accord struck by Russia, Ukraine, the United States and European Union in Geneva last week, illegal armed groups are supposed to disarm and go home, including rebels occupying about a dozen buildings in the largely Russian-speaking east.
The rebels have shown no sign of retreating.
"As President Obama reiterated earlier today, we are ready to act," Kerry said on Thursday.
When he announced the Geneva agreement a week ago, the U.S. secretary of state had said that if Russia did not take steps "over the course of these next days" there would be additional sanctions as a consequence.
Several sources said that the United States did not, however, wish to act on its own.
Washington's basic reasoning is that the practical effect on Russia's economy and markets, as well as the symbolic import of further sanctions, will be greater if the United States is seen to be acting in concert with the European Union.
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.
On March 21, the EU imposed sanctions on 12 Russians and Ukrainians because of Moscow's takeover of Crimea, bringing the number of people targeted by EU asset freezes and travel bans to 33.
On April 14, the EU agreed to expand the list of people subject to such penalties but the bloc has yet to agree on the names or to actually impose the sanctions.