ZAPORIZHZHIA, Ukraine (AP) — Russia resumed pulverizing the Mariupol steel mill that has become the last stronghold of resistance in the bombed-out city, Ukrainian fighters said Monday, after a brief cease-fire over the weekend allowed the first evacuation of civilians from the plant.

Meanwhile, a senior U.S. official warned that Russia is planning to annex large portions of eastern Ukraine this month and recognize the southern city of Kherson as an independent republic.

Michael Carpenter, U.S. ambassador to the Organization for Security and Cooperation in Europe, said that those suspected actions are “straight out of the Kremlin’s playbook” and will not be recognized by the United States or its allies.

In Mariupol, more than 100 people — including elderly women and mothers with small children — left the rubble-strewn Azovstal steelworks on Sunday and set out in buses and ambulances for the Ukrainian-controlled city of Zaporizhzhia, about 140 miles (230 kilometers) to the northwest, according to authorities and video released by the two sides.