Russia has distributed gas masks to troops and local militants in occupied Donetsk Oblast in preparation for a new strike, claims Ukraine Security Service.
It's feared Russian troops may be preparing to blow up chemical containers, according to the Kyiv Independent.
Donetsk is in the east of the country and is one of two areas first occupied by Russian insurgents earlier this week.
As the invasion continues for a third day, Ukrainian forces have clashed with Russian troops after a night of heavy shelling and missile strikes.
It comes as Ukraine denied suggestions that it was refusing to negotiate a ceasefire with Russia but said it was also not ready to accept ultimatums or unacceptable conditions.
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Mykhailo Podolyak, an adviser to the office of Ukrainian President Volodymyr Zelensky, said Ukraine has prepared a negotiating position but was faced with impractical
negotiating conditions from Russia.
"It was yesterday that the aggressive actions of the armed forces of the Russian Federation escalated, up to evening and night mass air and missile strikes on Ukrainian cities," he said in a message. "We consider such actions only an attempt to break Ukraine and force it to accept categorically unacceptable conditions."
As the invasion continues at least 198 Ukrainians, including three children, have been killed as a result of the Russian invasion of Ukraine, according to the head of the Ukrainian Health Ministry.
A defiant Zelensky remains in Kyiv as Russian troops advance towards the centre of the capital city.
The city remains under Ukrainian control and local officials have extended a curfew until 8am as air raid sirens blare and forces clash with Putin's troops.
Russia's defence ministry claimed its troops had captured the southeastern Ukrainian city of Melitopol earlier today but the UK's armed forces minister James Heappey said Britain did not believe Russian forces had captured the city.
Earlier today 60 Russian troops retreated from Lviv as Ukrainian forces defended the region.
Putin's troops are believed to have sustained greater casualties than anticipated as Ukraine mounts a fierce resistance.
