KYIV, April 12 - The battle for Mariupol appeared on Tuesday to be reaching a decisive phase, with Ukrainian marines holed up in the Azovstal industrial district. Reuters journalists accompanying Russian-backed separatists saw flames billowing from the Azovstal district, Reuters reported.

Pavlo Kyrylenko, governor of the eastern Donetsk region, which includes Mariupol, said he had seen incident reports on possible chemical weapons use in the city but could not confirm them.

"We know that last night around midnight a drone dropped some so-far unknown explosive device, and the people that were in and around the Mariupol metal plant, there were three people, they began to feel unwell," he told CNN.

They were taken to hospital and their lives were not in danger, he said.

Late on Tuesday, Ukraine said its forces in the east had beaten off six Russian attacks, destroying two vehicles and three artillery systems as well as shooting down a helicopter and two drones. Reuters could not immediately verify the report.

Zelenskiy had said on Monday that Russia may resort to chemical weapons as it massed troops for a new assault. He did not say if they actually had been used. The United States and Britain said they were trying to verify the reports.

Chemical weapons production, use and stockpiling is banned under the 1997 Chemical Weapons Convention.

Russia's defence ministry has not responded to a Reuters request for comment. Russian-backed separatist forces in the east denied using chemical weapons in Mariupol, the Interfax news agency reported.

After their troops got bogged down in the face of Ukrainian resistance, the Russians abandoned their bid to capture Kyiv and other major Ukrainian cities, focusing its attacks on eastern regions of Donbas, which includes parts of Donetsk and Luhansk regions. (rt/ez)