BEIJING, April 19 (Reuters) - Russia on Tuesday called on Ukrainian forces and foreign fighters holed up in the Azovstal metallurgical plant in the Ukrainian port city of Mariupol to lay down their arms by noon Moscow time (0900 GMT) if they wanted to live.

Mariupol, which has been encircled by Russian troops for weeks, has seen the fiercest fighting and most comprehensive destruction since Russia sent troops into Ukraine on Feb. 24.

Ukrainian authorities said on Monday that no fewer than 1,000 civilians were hiding in underground shelters beneath the vast Azovstal plant, adding that 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 1400 and 1600 Moscow time "without exception, without any weapons and without ammunition".

Earlier on Tuesday, Russia-backed separatist forces said they were trying to storm the Azovstal steel worksand take control of it as quickly as possible.