Obama Calls on Russia to Call Off Militants in Eastern Ukraine

U.S. President Barack Obama on Tuesday called on Russia to pressure separatists to stop attacking government forces in eastern Ukraine and to prevent the flow of fighters and weapons into the region.

Obama made the call in a press conference in Poland and also warned that further Russian action in Ukraine could lead to stepped-up economic sanctions.

"Further Russian provocation will be met with further costs for Russia including, if necessary, additional sanctions," Obama said.

The president, who will meet Ukrainian president-elect Petro Poroshenko in Warsaw on Wednesday, called on Russia to use its influence to "prevent the flow of militants and weapons into eastern Ukraine."

He said President Vladimir Putin's government should also convince pro-Kremlin separatists to "stop attacking Ukrainian security forces."