President Obama spoke Tuesday with the leaders of both Russia and Ukraine, one day before they meet for talks aimed at reaching an elusive peace deal.
The White House said Obama reiterated U.S. support for Ukraine's sovereignty in his call with Russian President Vladimir Putin, and emphasized the importance of reaching a diplomatic resolution.
"However, if Russia continues its aggressive actions in Ukraine, including by sending troops, weapons, and financing to support the separatists, the costs for Russia will rise," the White House said in a statement.
Obama also discussed Wednesday's cease-fire talks in Minsk, Belarus, with Ukrainian President Petro Poroshenko.
Germany and France are leading the talks aimed at ending the conflict in eastern Ukraine, where violence between government forces and Russian-backed rebels has escalated in recent weeks. The U.S. is not formally part of the discussions.
Obama met at the White House Monday with German Chancellor Angela Merkel and both leaders rallied around the long-shot diplomatic effort.
The president said that he is also weighing sending defensive lethal aid to bolster the beleaguered Ukrainian military.