Russia's Azovstal ultimatum: submit or die

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.