Russia has called on Ukrainian forces and foreign fighters holed up in the Azovstal metallurgical plant in the port city of Mariupol to lay down their arms within hours if they want to live.
Mariupol, which has been encircled by Russian troops for weeks, has seen the fiercest fighting and most comprehensive destruction since Vladimir Putin ordered his troops into Ukraine on February 24.
Ukrainian authorities said on Monday at least 1000 civilians were hiding in underground shelters beneath the vast Azovstal plant, adding Russia was dropping heavy bombs onto the Ukrainian-held factory in the besieged city.
Russia's defence ministry on Tuesday issued a statement calling on Ukrainian forces and foreign fighters inside to surrender.
"All who lay down their arms are guaranteed to remain alive," the defence ministry said.
It called on troops to withdraw from the steel plant between 2pm and 4pm Moscow time (9pm and 11pm AEST) "without exception, without any weapons and without ammunition".
Earlier on Tuesday, Russia-backed separatist forces said they were trying to storm the plant and take control of it as quickly as possible.
Australian Associated Press
