President Obama spoke with Russian President Vladimir Putin and Ukrainian President Petro Poroshenko the day before planned peace talks.

The talks among Russia, Ukraine, Germany and France are scheduled for Wednesday in Minsk. They seek to revive a long-stagnant peace plan signed in September in the Belarusian city, and come amid heavy fighting in eastern Ukraine.

The White House said Tuesday that Obama stressed the "importance of reaching and implementing a negotiated settlement underpinned by the commitments in the Minsk agreement" and warned that 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."

Poroshenko and Obama said Russia needs to stand by the commitments it made in September, which should remain the basis for a settlement. Obama reiterated his support for a peace deal.

Obama met with German Chancellor Angela Merkel on Monday. The president said he would consider providing defensive arms to Ukraine should a peace deal fail, a stance that Merkel opposes. Both sides said the United States and Europe remain united in trying to achieve a diplomatic end to the crisis.

Obama said he and Merkel agreed that “sanctions on Russia need to remain fully enforced until Russia fully complies with its obligations” on the peace treaty. In Brussels on Monday, the European Union delayed imposing a new round of sanctions against Russia ahead of the talks Wednesday.

"And as diplomatic efforts continue this week, we are in absolute agreement that the 21st century cannot . . . have us stand idle and simply allow the borders of Europe to be redrawn at the barrel of a gun," Obama said.