The United States will keep extra forces in Europe to bolster NATO defenses and monitor new movements of Russian tanks in east Ukraine as a cease-fire deadline approaches, officials said Thursday.

Under the terms of the peace agreement, Ukrainian forces and Russian-backed separatists are to reach a full cease-fire by 12:01 a.m. Sunday followed by the complete withdrawal of all heavy weapons 14 days after the cease-fire. The terms were agreed to late Wednesday and into early Thursday even as reports came in that Russia moved a column of 50 tanks into east Ukraine.

“We are concerned about reports that the fighting continues,” said a senior Obama administration official. “We are also concerned about reports of new heavy weapons transferred into Ukraine.”

The movement of Russian tanks, if it constitutes a last-minute land grab prior to the cease-fire, “would seriously undercut this agreement,” the official said.

Any such maneuver would “be a clear violation and would cause us and our partners to impose more costs,” another official said, noting that economic sanctions and the possibility of providing lethal weapons to Ukraine remain “very much on the table."

In the last week, the Pentagon has announced several moves to shift resources to the region. On Saturday, it announced that the Arleigh Burke class destroyer USS Cole entered the Black Sea to conduct patrols “in light of the ongoing Russian intervention in Ukraine.”

On Wednesday, the Pentagon said one dozen A-10 Thunderbolt II attack aircraft and 300 support personnel and equipment from the 355th Fighter Wing at Davis-Monthan Air Force Base, Ariz., would deploy to Spangdahlem Air Base in Germany. The unit is slated to remain in Germany for six months, “dependent on the mission and European Command requirements,” said Air Force spokesman Lt. Col. Chris Karns.

The Pentagon also announced that an unspecified number of soldiers from the 173rd Airborne Brigade assigned to U.S. Army Europe would train Ukraine’s National Guard forces, who are assigned to protect the country’s infrastructure, “to strengthen Ukraine’s internal defense capabilities.”

On Thursday, the Pentagon said no adjustments would be made to those plans.

“Our Atlantic Resolve operations continue apace,” said Pentagon spokesman Col. Steve Warren. “There have been no changes.”

In an earlier interview, the Commander of U.S. Army Europe, Lt. Gen. Ben Hodges said the forces from the 173rd would help Ukrainian Ministry of the Interior troops guarding the country's infrastructure to evade and survive their better-equipped foes.

“Obviously they are going to be targeted by artillery, they are going to be detected, they are going to be jammed, so we’re [going to] help them to be able to operate in that environment,” Hodges said.