The Ukrainian Ministry of Defense says that Russian forces are trying to develop their offensive in the Luhansk region with "continuous attempts" to cross the Siverskyi Donets river near Belahorivka.
Col. Oleksandr Motuzyanyk, the spokesperson for the Ukrainian Defense Ministry, said the Russians had built three pontoon crossings across the river and were supporting ground troops with artillery and aircraft.
He said the Russians were aiming to cut off Lysychansk, a town on the frontlines some ten miles (about 16 kilometers) from one of the pontoon bridges identified on satellite imagery.
If successful, the Russian advance might be able to cut Ukrainian supply lines to the defenders of Lysychansk and Severodonetsk.
But images reviewed by CNN indicate that at least one pontoon bridge has already been destroyed, with ruined tanks and other equipment half-submerged in the river.
"Our troops are clearing the territory of Belohorivka, where the occupiers crossed the river by pontoon crossing," Serhiy Hayday, the head of the Luhansk military administration, said.
"After the clean-up of Belohorivka, evacuation will resume," he added, saying rescuers would be able to clear the rubble of the school building that was hit on Saturday by a bomb dropped by a Russian aircraft, killing dozens of people taking refuge in the school.
It's unclear whether the Ukrainians retain control of Belohorivka.
Hayday said that in Popasna —to the south — "our defenders keep the defense in new fortified positions, there are no breakthroughs. Soon the situation must change in our favor."
More context: Ukrainian officials have raised the alarm about a Russian pontoon bridge that was erected a few days ago and may now enable Russian forces to threaten Ukrainian defenses and supply routes in the Luhansk region.
Hayday said earlier Monday that the Russians were trying to take away "the road of life," which connects the front lines in the east around Severodonetsk with the town of Bakhmut, an important rear base.
CNN had confirmed satellite imagery showing a pontoon bridge across the Siverskyi Donets on Sunday. There had been no such bridge on May 3.
The location of the bridge is less than two miles (about 3 kilometers) from the village of Bilohorivka, which was bombed by Russian aircraft on Saturday.
CNN's Gianluca Mezzofiore contributed reporting to this post.