NATO Secretary General Jens Stoltenberg has called on Russia to use its "considerable influence" to end fighting in eastern Ukraine after a renewed surge in violence there.
Moscow-backed rebels and government forces have trade blame for the flare-up in the industrial east that has seen the highest casualty rate since mid-December and cut off power and water to thousands of civilians on both sides of the front line.
The shelling eased on Wednesday, but the January 29 to 31 clashes near the Kiev-held front line town of Avdiyivka brought the festering conflict back into focus amid warnings of a looming humanitarian crisis.
"We call for an immediate return to the ceasefire," Stoltenberg said in Brussels. "We call on Russia to use its considerable influence over the separatists to bring the violence to an end."
Ukraine and NATO accuse Russia of providing troops and weapons to support separatist rebels in the country's east in a conflict that has killed 10,000 people since April 2014. Moscow denies that.
Michael Bociurkiw, a spokesman for the European security body OSCE's mission to Ukraine, told Al Jazeera that the new US administration was not being clear about how it would deal with the rising tensions.
"In the previous administration, Russians were really attacked for their activities in eastern Ukraine but the Trump administration is really sending mixed signals," he said.
"The big fear is that Trump may withdraw support for the sanctions against Russia and that could turn a chain reaction where Greece, Italy and other EU members may also withdraw support."