Russia launched an attack on the encircled Azovstal steel works in Mariupol, Ukraine’s last redoubt in the port city, after a ceasefire broke down on Tuesday with some 200 civilians trapped underground despite a UN-brokered evacuation.

In a Telegram video, Captain Sviatoslav Palamar of Ukraine’s Azov Regiment said that Russia pounded the steel works with naval and barrel artillery through the night and dropped heavy bombs from planes.

Reuters could not independently verify his account. However, Reuters images on Monday showed volleys of rockets fired from a Russian truck-mounted launcher towards Azovstal, a sprawling Soviet-era steel works.

“As of this moment, a powerful assault on the territory of the Azovstal plant is under way with the support of armoured vehicles, tanks, attempts to land on boats and a large number of infantry,” Palamar said. He added that two civilians were killed and 10 injured, without providing evidence.

Russia has turned its fire power on Ukraine’s east and south after failing to take the capital of Kyiv in the north in March. The offensive has been met with commitments by western powers for tougher sanctions as well as supplies of heavier weapons to Ukraine, including air defence systems and long-range artillery.

On Tuesday, the European Commission was expected to finalise a ban on buying Russian oil in an effort to squeeze Moscow’s war chest. The US Congress is considering a $33 billion military aid package, and the UK this week vowed an additional $375 million in defence assistance.

“This is Ukraine’s finest hour, (one) that will be remembered and recounted for generations to come,” British Prime Minister Boris Johnson said in an address to Ukraine’s parliament via videolink. He was channelling the words spoken by Winston Churchill in 1940 when Britain faced the threat of being invaded and defeated by Nazi Germany.