Washington has called on the parties to the conflict in Ukraine to adhere to the cease-fire.

"We continue to call on all sides to strictly adhere to the cease-fire," the BBC quoted NSC Spokesperson Bernadette Meehan as saying on Monday.

She noted that "any attempt by separatist forces to seize additional territory in eastern Ukraine would be a blatant violation of the Minsk agreements."

"We are very concerned by intensified fighting in eastern Ukraine, as well as numerous reports, including from the Special Monitoring Mission of the Organization for Security and Cooperation in Europe (OSCE), that Russian backed and supplied separatists are moving large convoys of heavy weapons and tanks to the front lines of the conflict," she said.