A Ukrainian military commander said that Russia is dropping bombs on a steel plant in Mariupol, where soldiers and citizens have been hiding.
The commander of the Ukrainian National Guard said on Monday that Russia had started dropping bombs on the Azovstal Iron and Steel Works plant.
Prokopenko said his troops were there.
The city council of Mariupol said on Telegram that up to 1,000 citizens were hiding there.
Bunker busting bombs are designed to hit targets that are deep underground.
Russian occupational forces and their proxy were aware that civilians were hiding and were willing to fire on the factory.
The defense ministry of Russia told troops in the plant to lay down their arms by midday Moscow time.
Children were among the civilians at the plant, according to the head of the regional patrol police. Mariupol is in the eastern part of the country.
Mariupol is one of the most-hit parts of the country. Russia would have effective control over the land route from Russian-controlled Crimea and the eastern Donbas region if Mariupol were to be Seized.
The city doesn't exist anymore, as Ukrainian Foreign Minister Dmytro Kuleba said on Sunday. The Russian forces encircled the Ukrainian army and a large group of civilians.
The steel plant may be the last pocket of resistance in the city.
Ukrainian soldiers in Mariupol have refused to surrender.
The steel plant under Mariupol is basically another city and was built to survive bombings, according to an advisor to the Russia-backed rebels.