The loss of a final bridge to Sievierodonetsk may hurt Ukraine’s ability to hold its ground.

As Ukrainian and Russian soldiers engage in street battles in Sievierodonetsk and scores of civilians remain stranded under unrelenting bombardment, the last bridge linking the eastern city with Ukrainian-controlled territory to the west has been destroyed, according to officials from both nations.

The destruction of the bridge could help Moscow seize the city by making it harder for Ukrainian soldiers to reinforce their positions, evacuate their wounded or retreat. While Ukrainian officials say that supplies can still get into and out of Sievierodonetsk, the loss of the bridge, after weeks of fighting there, makes it unclear whether Ukrainian forces can continue to defend the city.

The bridge’s destruction all but cuts the final road link between Sievierodonetsk, which lies on the eastern bank of the Siversky Donets River, and the Ukrainian-controlled city of Lysychansk, which lies on higher ground to the west. It could also complicate the Russian advance, making any frontal assault on Lysyschansk more difficult, as it would require the Russians to expose their troops to attack while crossing the river.

It remains unclear who destroyed the bridge, and the Ukrainians did not specify how they would move supplies into Sievierodonetsk, leaving open the possibility that boats or pontoon bridges could be used. Volunteers are trying to collect boats and ropes for Ukrainian troops in the city.

The intensity of the fighting in Sievierodonetsk reflects its importance as the biggest city in Luhansk Province not yet under Moscow’s control. In 2014, Russian-backed separatists seized much of the Donbas region, which is made up of Luhansk and Donetsk Provinces. Taking complete control of Donbas became President Vladimir V. Putin’s military priority after his forces failed to seize the Ukrainian capital, Kyiv, or the country’s second largest city, Kharkiv.

Other smaller bridges, to the north and south of Sievierodonetsk, had previously been destroyed by Russia or put out of action by Ukrainian troops hoping to impede Russia’s advance. The latest bridge to be destroyed was the city’s main span, connecting to the main P66 road heading southwest out of the city. Even before it was destroyed, the bridge had been littered with damaged and abandoned vehicles and was only partly passable.

The river is about 50 yards wide as it runs through the city. Both banks of the river are targeted often and have been the site of heavy shelling in recent days.

For weeks, Russian forces have assaulted Sievierodonetsk from the east, north and south, pounding it with artillery in an attempt to drive Ukrainian troops toward the river to the west. In fierce street fighting, they took a hotel and bus station in the northeast of the city, Ukrainian officials said, and have then fought their way toward the center.

Russian forces have also been making targeted attacks on the city’s Azot chemical plant, where 500 civilians have been sheltering, Serhiy Haidai, the regional governor of Luhansk Province, where Sievierodonetsk is, said on national television on Tuesday. He also confirmed that the last bridge to the city had been destroyed.

The mayor of Sievierodonetsk, Oleksandr Stryuk, said on national television that the situation was “very difficult.”

“Russian troops are trying to storm the city, but the military is holding firm,” he said, adding that despite the lack of bridges, civilian evacuations were still being carried out.

At the same time, Russian forces have shelled the highway that runs between Lysychansk and the city of Bakhmut to the southwest in an attempt to cut Ukrainian supply lines to the front. Ukrainian officials have called that route the “road of life” and have said for more than a week that it has been too dangerous to use.

Natalia Yermak contributed reporting.