Colonel Gen. Mikhail Mizintsev gave Ukrainian troops holed up at the giant Azovstal steel plant in Mariupol until 12:00 PM on Tuesday (0900 GMT) to surrender.
He said those who surrender will “keep their lives.”
Ukrainian troops who have defended the city for seven weeks have ignored such previous offers. The Azovstal factory, which covers an area of about 11 square kilometers, is the last major Ukrainian center of resistance in Mariupol, a strategic port on the Sea of Azov.
Earlier Tuesday, Eduard Basurin, a spokesman for the Russian-backed separatists in the Donbas region, said assault groups had entered Azovstal in an attempt to wipe out Ukrainian forces after bombing and artillery shelling.