Siversk, which had a pre-war population of 10,000, is 35 kilometres north of Bakhmut - scene of fierce fighting in recent weeks - and on a direct road to another of the key towns in the Donetsk region, Sloviansk.
"If they occupied Bakhmut, then we would be semi-encircled, because on the left side we have the Siverskyi Donets river, and the enemy will advance from the right, and it is possible to cut us off if they reach the Bakhmut highway," said the deputy Siversk battalion commander, who used the nom-de-guerre "Han".
Ukrainian forces have appealed for more advanced weapons from Kyiv's Western allies to help defend Bakhmut, which is being attacked by Russia's Wagner mercenary group.
One soldier defending Siversk said their mostly Soviet-era artillery was outgunned by the enemy.
"We have one artillery attack from our side, and the Russians can do it five times more," said 30-year-old Stefan. "It's very difficult for the guys who are standing, especially at the first line (of defence), they feel it too much."
Capturing Bakhmut would give Russian forces a stepping stone to advance on two bigger cities further west, Kramatorsk and Sloviansk.
But Ukraine and its Western allies say success there would be a largely Pyrrhic victory for Moscow, given the time taken and losses sustained.
Elsewhere in eastern Ukraine, several Russian shells have hit the town of Druzhkivka, according to local authorities.
In western Ukraine, a long-range rocket hit a military object in the capital of Khmelnytskyi province, according to authorities. Another missile hit near a holding area. The blast wave damaged several homes and schools.
On Sunday morning, there was an intermittent air alert in eastern and southern Ukraine, with no reports of actual attacks.
The Russian war against the neighbouring country has been going on for almost one year.
with DPA
