The two countries appear to be trying to raise political pressure on each other and send signals to Washington in case peace talks move forward.

Oil prices edged higher on Monday after Ukraine stepped up attacks on Russia, fanning concerns Russian oil supply could be disrupted, while expectations of a cut in U.S. interest rates buoyed the outlook for global growth and fuel demand.

Momentum toward peace in Ukraine appeared to stall over the weekend after Russia launched another round of strikes.