(QHA) -

U.S. President Barack Obama has urged Russian President Vladimir Putin to support peace in Ukraine and said Moscow would face further "costs" if it did not take steps to reduce tensions in the country, Reuters reports.

The White House said Obama made his comments to Putin via phone today, calling on him to halt the flow of "arms and materiel across the border" and Kremlin support for "militants and separatists."

Obama added that Russia will face additonal sanctions if Washington does not see "concrete actions to de-escalate the situation."

Putin, for his part, was said to have emphasized to Obama the importance of direct talks between Ukrainian officials and the pro-Russian separatists in eastern Ukraine.

An unilateral cease-fire declared by Ukrainian President Petro Poroshenko last week was agreed to earlier today by some pro-Russian separatist leaders after talks chaired by former Ukrainian President Leonid Kuchma.