By Natalia Zinets and Abdelaziz Boumzar

KYIV/BAKHMUT, Ukraine (Reuters) – Russian forces pushed into the eastern Ukrainian city of Sievierodonetsk and pounded a zone where hundreds of civilians were sheltering, a Ukrainian official said on Monday – a scene that echoed Moscow’s assault on Mariupol last month.

Pro-Moscow separatists said the last bridge out of Sievierodonetsk had been destroyed, cutting off the defenders’ escape route. Ukraine said there was still another way out, although that route was severely damaged.

Ukraine has issued increasingly urgent calls for more Western heavy weapons to help defend Sievierodonetsk, which Kyiv says could hold the key to the battle for the eastern Donbas region and the course of the war, now in its fourth month.

“Apart from constant artillery, aircraft and rocket bombing, there is a real risk that the defenders in Sievierodonetsk might be cut off from Lysychansk if the third bridge linking the two cities is destroyed,” said Damien Magrou, spokesperson for the International Legion for the Defence of Ukraine that has had forces in Sievierodonetsk.

“And then you’re potentially looking at a situation comparable with that of Mariupol with a large pocket of Ukrainian defenders cut off from the rest of the Ukrainian troops. This is one of the reasons why it is so important that our Western partners deliver long range artillery as fast as possible.”

Regional governor Sergei Gaidai said on social media that Russian forces now controlled about 70% of Sievierodonetsk, and were destroying it “quarter by quarter”.

“The battles are so fierce that fighting for not just a street but for a single high-rise building can last for days,” said Gaidai, who is governor of the Luhansk region that includes Sievierodonetsk.

Russian artillery fire pummelled the Azot chemical plant, where hundred of civilians were sheltering, he said.

“About 500 civilians remain on the grounds of the Azot plant in Sievierodonetsk, 40 of them are children. Sometimes the military manages to evacuate someone,” he said.

‘SURRENDER OR DIE’

Russia’s RIA news agency quoted a pro-Moscow separatist spokesperson, Eduard Basurin, as saying the last bridge over the Siverskyi Donets river linking Sievierodonetsk and its Ukrainian-held twin city Lysychansk had been destroyed on Sunday. Ukrainian troops were effectively blockaded in Sievierodonetsk and should surrender or die, he said.

Ukraine’s account of civilians trapped in an industrial plant echoed the fall of Mariupol last month, where hundreds of civilians and badly wounded Ukrainian soldiers were trapped for weeks in the Azovstal steelworks.

Russia has denied targeting civilians in what it calls a “special operation” to restore Russian security and “denazify” its neighbour. Ukraine and its Western allies call this a baseless pretext for an invasion which has killed thousands of civilians and raised fears of wider conflict in Europe.

More than 5 million people have fled the assault and millions more are threatened by a global energy and food crisis due to disrupted gas, oil and grain supplies from Russia and Ukraine. Western nations are divided over how best to end it.

Gaidai said a six-year-old child was among those killed in the latest shelling of Lysychansk. Officials in the Russian-backed separatist-controlled Donetsk region said at least three people, including a child, were killed and 18 were wounded by Ukrainian shelling that hit a market in Donetsk city.

The Donetsk News Agency showed pictures of burning stalls at the central Maisky market and several bodies on the ground. The news agency said 155-mm calibre NATO-standard artillery munitions hit parts of the region on Monday.

Reuters could not independently verify either report.

BURNING CROPS

After failing to take the capital Kyiv following the Feb. 24 invasion, Moscow focused on expanding control in the Donbas, which comprise Luhansk and neighbouring Donetsk and where pro-Russian separatists have held territory since 2014, while also trying to capture more of Ukraine’s Black Sea coast.

Along the front line in the Donbas, the fighting poses a new threat as the weather warms, with shelling and rocket fire setting fields on fire and destroying ripening crops.

Lyuba, a resident in the Ukrainian-held pocket of the Donbas near the front, watched a fire blazing along the fields but said she was not planning to leave. “Where can I go? Who is waiting for me there?” she said. “It’s scary. But it is what it is.”

Ukrainian Presidential Adviser Mykhailo Podolyak listed equipment he said was needed for heavy weapons parity, including 1,000 howitzers, 500 tanks and 1,000 drones.

“We are waiting for a decision,” he said, adding that Western defence ministers would meet on Wednesday in Brussels.

Russia issued the latest of several recent reports saying it had destroyed U.S. and European arms and equipment, hoping to send the message that delivering more would be futile.

The defence ministry said high-precision air-based missiles had struck near the railway station in Udachne northwest of Donetsk, hitting equipment that had been delivered to Ukrainian forces. There was no immediate word from the Ukrainian side.

Moscow has criticised the United States and other nations for sending Ukraine weapons, threatening to strike new targets if the West supplied long-range missiles.

(Reporting by Reuters bureaux; Writing by Michael Perry, Philippa Fletcher and Alex Richardson; Editing by Simon Cameron-Moore, Peter Graff and Toby Chopra)