All bridges to the Ukrainian city of Severodonetsk are now impassable, making it impossible to get civilians out and humanitarian aid in.
Serhiy Haidai, governor of the Luhansk region, which includes Severodonetsk, said that Russians had destroyed two of the three bridges connecting the city with Lysychansk, and the third is old and unsafe.
Mr Haidai said: “It is now fully impossible, unfortunately, to drive into the city, to deliver something to the city.
“Evacuation is impossible.”
He estimated that about 70% of Severodonetsk is in Russian hands, adding that the situation for Ukrainian troops is “difficult but under control”.
Mr Haidai told RFE/RL’s Ukrainian service: “They have the ability to send the wounded to hospitals, so there is still access.
“It is hard to deliver weapons or reserves – difficult, but not impossible.”
He also told The Associated Press that Ukrainian forces are fighting the Russians “block by block, street by street, house by house, with a varying degree of success.”
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More than 10,000 civilians remain in Severodonetsk – from a pre-war population of more than 100,000.
Lysychansk is the other city in Luhansk not yet under Russian control but it is being regularly shelled by Russian forces.
Eduard Basurin, an official of the Russian-backed separatists in neighbouring Donetsk, said on Monday that Severodonetsk had been blocked off and Ukrainian fighters have no choice but to surrender.
Mr Haidai dismissed this is “a lie”, however, adding: “There is no threat of our troops being encircled in the Luhansk region.”
Russia invaded Ukraine on 24 February but failed to advance as quickly as expected, abandoning efforts to take the capital Kyiv and shifting its focus to the Donbas region, where Moscow-backed separatists have held territory since 2014.
On Monday, Kremlin spokesman Dmitry Peskov said that Russia’s main goal is to protect the Donetsk and Luhansk people’s republics, which make up the Donbas region.
He told Russia’s RIA news agency: “In general, the protection of the republics is the main goal of the special military operation.”
Ukraine continues to plead with western powers for more weapons to help defend its remaining territory in the Donbas.
Ukrainian Presidential Adviser Mykhailo Podolyak said 1,000 howitzers, 500 tanks and 1,000 drones were among the items needed for his country to be on an equal footing with Russia in terms of weapons.
Meanwhile, Russia said it had destroyed another shipment of US and European weapons and equipment in a missile strike near a railway station in Udachne, northwest of Donetsk.