A Russian ultimatum to Ukrainian troops in Mariupol to surrender or die expired on Wednesday afternoon with no mass capitulation, but the commander of a unit believed to be holding out in the besieged city said his forces could survive just days or hours.

Thousands of Russian troops backed by artillery and rocket barrages were attempting to advance elsewhere in what Ukrainian officials call the Battle of the Donbas – a push by Moscow to seize two eastern provinces. 

In a video, the commander of Ukraine's 36th Marine Brigade, one of the last units believed to be holding out in Mariupol, asked for international help to escape the siege.

"This is our appeal to the world. It may be our last. We may have only a few days or hours left," said Major Serhiy Volyna in a video uploaded to Facebook. "The enemy units are dozens of times larger than ours, they have dominance in the air, in artillery, in ground troops, in equipment and in tanks. 

Russia said shortly before a 2 p.m. (1100 GMT) Wednesday deadline that just five people had surrendered. The previous day, Russia said no one had responded to a similar surrender demand.

The United Nations said on Wednesday the number of refugees who have fled Ukraine since Febuaury 24 had surpassed five million. More than half are children.

Ukraine announced plans to send 90 buses to evacuate 6,000 civilians from Mariupol, saying it had reached a "preliminary agreement" with Russia on a safe corridor for the first time in weeks. But none of those earlier agreements have actually succeeded on the ground, with Moscow blocking all convoys.

Once a prosperous port of 400,000 people, Mariupol has been reduced to a blasted wasteland with corpses in the streets and residents confined to cellars. Tens of thousands of civilians have died there, according to Ukraine officials.

Battle of Donbas

The battle for the Donbas region, which includes the provinces Luhansk and Donetsk, could be decisive for the conflict between Russia and Ukraine. Putin accuses Ukraine of mistreating Russian-speakers in the Donbas, which Kyiv and its Western allies call a falsehood to justify an unprovoked land grab.

Russian television showed Putin addressing a girl from Luhansk on Wednesday: "It was the tragedy that took place in the Donbas, including in the Luhansk People's Republic, that forced, simply forced Russia to launch this military operation, which everyone is well aware of today," he said.

Peace talks have been stalled. The Kremlin accused Kyiv of delaying the talks and changing its positions. Kyiv accuses Moscow of blocking talks by refusing humanitarian cease-fires, especially to relieve besieged Mariupol. 

British military intelligence said fighting in the Donbas was intensifying as Russian forces tried to break through Ukraine's lines, and that Russia was still building up forces on Ukraine's eastern border.

(With input from agencies)