Washington: The White House expressed grave concern Sunday at reports of Russian military reinforcements in eastern Ukraine, warning that any separatist efforts to seize more territory would be a "blatant violation" of a ceasefire agreement.
Armored convoys headed to the frontlines as shelling rocked separatist stronghold Donetsk and fears mounted of a return to full-scale fighting.
The shelling in Donetsk was among the fiercest combat in the city since the September 5 signing of a frequently-violated ceasefire that halted all-out confrontations across most of the conflict zone but failed to end constant bombardments at strategic hotspots.
A convoy of 20 military vehicles and 14 howitzer cannons without number plates or markings was seen driving through the rebel town of Makiivka in the direction of the nearby frontline around Donetsk.
"We are very concerned by intensified fighting in eastern Ukraine, as well as numerous reports... that Russian backed and supplied separatists are moving large convoys of heavy weapons and tanks to the front lines of the conflict," National Security Council spokeswoman Bernadette Meehan said in a statement.
"We continue to call on all sides to strictly adhere to the ceasefire. Any attempt by separatist forces to seize additional territory in eastern Ukraine would be a blatant violation of the Minsk agreements."
Meehan urged Moscow to honor the commitments it made when signing the ceasefire agreement, including halting its military supplies to separatists and the withdrawal of all its troops and weapons from Ukraine.
"Russia must enable the restoration of Ukrainian sovereignty along the Ukrainian side of the international border, to be monitored by the OSCE, and facilitate the release of all hostages," she said, referring to the Organization for Security and Cooperation in Europe.
"We continue to stress that adherence to the framework agreed upon in Minsk is the best chance of achieving a peaceful resolution to the conflict in eastern Ukraine."
Russia denies being involved in the fighting in the east.
The OSCE voiced concern Saturday after its monitors witnessed unmarked columns of tanks and troop carriers moving through east Ukraine in territory held by pro-Russia separatists.
The sightings of armored columns came after Ukraine`s military said Friday a large convoy of tanks and other heavy weapons entered the country from Russia across a section of border that has fallen under the control of rebel fighters.