“President Obama underscored the rising human toll of the fighting and underscored the importance of President Putin seizing the opportunity presented by the ongoing discussions between Russia, France, Germany, and Ukraine to reach a peaceful resolution,” a brief communique issued by the White House said.

The U.S. president also repeated to Putin that America continues to support “the sovereignty and territorial integrity of Ukraine.”

Consequently, he said 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,” a reference to the economic sanctions imposed by the European Union and the United States.

The president’s call came shortly after Ukraine and the pro-Russian separatists agreed Tuesday to a cease-fire and the withdrawal of heavy armaments during the peace negotiations being held in Minsk, Belarus, with the mediation of Russia and the Organization for Security and Cooperation in Europe, or OSCE.

These conversations are the prelude to the summit meeting of Ukraine, Russia, Germany and France to be held this Wednesday in the Belarus capital, considered of key importance for finding a lasting peace.

Hours before speaking with Putin, the U.S. president had conversed with Ukrainian President Petro Poroshenko, to whom he extended his strong support for the current peace negotiations.