International law prohibits the use of white phosphorus shells in heavily populated civilian areas but allows them in open spaces to be used as cover for troops.

Oleksi Biloshytsky, head of police in Popasna, around 100 kilometres (60 miles) west of Lugansk city, said late on Saturday that Russian forces had used the chemical weapon in his area.

"It's what the Nazis called a 'flaming onion' and that's what the Russcists (amalgamation of 'Russians' and 'fascists') are dropping on our towns. Indescribable suffering and fires," he wrote on Facebook.

It was not immediately possible to verify the comments.

Further south, bombs struck the Sviatoguirsk monastery, where nearly 1,000 civilians were sheltering at the weekend, wounding 30 people, the Ukrainian public prosecutor said at the weekend.

And on Saturday night, a train evacuating people from the Donbas to the western city of Lviv was shelled, according to Donetsk military commander Pavlo Kirilenko.

One person was killed and another wounded, he said.

The Lugansk and Donetsk regions of eastern Ukraine, collectively known as the Donbas, were partially controlled by Moscow-backed separatists even before the full-scale Russian invasion of Ukraine on February 24.

The areas attacked at the weekend lie outside the so-called separatist "republics" of Lugansk and Donetsk declared by the pro-Russian rebels before the start of the war.

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