The White House said Obama underscored to Russian President Vladimir Putin in a phone call "the importance of ... seizing the opportunity presented by the ongoing discussions between Russia, France, Germany and Ukraine to reach a peaceful resolution" to the violence in Eastern Ukraine.

A day earlier, Obama said he hadn't yet decided whether or not to send arms and equipment to forces loyal to Kiev, saying he was still seeking a diplomatic end to the crisis. Standing alongside German Chancellor Angela Merkel at a press conference, Obama said the chances for a military solution in Ukraine were "low."

"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," Obama told Putin, according to the White House.

European leaders have pleaded with the United States to hold off sending lethal aid to Ukraine, despite calls from a bipartisan group of U.S. lawmakers and former administration officials backing shipments of arms to Kiev's besieged forces.

Merkel, along with French President Francois Hollande, met with Putin in Moscow late last week to try and develop a peace accord, but walked away with few results.

The leaders will try again to reach a deal Wednesday in Minsk.

(CNN)