NATO warns Russia over Ukraine
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."
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."