A Russian ultimatum to Ukrainian troops in Mariupol to surrender or perish expired on Wednesday afternoon without eliciting widespread submission, but the unit leader believed to be holding out in the beleaguered city indicated his forces may survive for just days or hours.

Thousands of Russian troops were seeking to advance further in what Ukrainian officials refer to as the Battle of the Donbas — a campaign by Moscow to conquer two eastern regions it claims on behalf of separatists.

The commander of Ukraine's 36th Marine Brigade, one of the last battalions believed to be defending Mariupol, appealed for international assistance in escaping the siege in a video.

"This is our message to the rest of the world. This may be our final meeting. We may just have a few days or hours left," Major Serhiy Volyna warned in a Facebook video. "Our adversaries' units are dozens of times larger than ours; they dominate the air, artillery, ground troops, equipment, and tanks."

Volyna, who has previously stated that women and children are being held captive in cellars beneath the factory, talked in front of a white brick wall in what seemed like a busy room. Reuters could not confirm the location or date of the videotape or who else may have been in the room.

According to the United Nations, the number of refugees fleeing Ukraine since Russia invaded on February 24 has topped five million. Over half of the population is comprised of children.

Russia's nearly eight-week invasion of Ukraine has failed to conquer any of the country's major cities. Moscow was forced to withdraw from northern Ukraine following a failed assault on Kyiv last month but has now poured troops back in for an assault on the east that began this week.

Russia was attacking the last major Ukrainian stronghold, the Azovstal steel complex, with bunker-buster bombs in the ruins of Mariupol, Kyiv reported.

"The world stands by and watches as children are murdered online," presidential adviser Mykhailo Podolyak posted on Twitter.

Russia has been attempting to seize complete control of Mariupol since the beginning of the war. Capturing it would be a significant strategic coup, connecting territory held by pro-Russian separatists in the east with the Crimea peninsula seized by Moscow in 2014.

Separatists backed by Russia announced that only five persons had surrendered just before Wednesday's 2 p.m. deadline. Russia had claimed that no one had responded to a similar surrender demand the previous day.

Ukraine announced preparations to send 90 buses to evacuate 6,000 residents from Mariupol, claiming for the first time in weeks that it had struck a "preliminary agreement" with Russia on a safe path. However, none of those previous agreements have been implemented, with Moscow obstructing all convoys.

Once a bustling port of 400,000 inhabitants, Mariupol has been turned into a blasted wasteland, with bodies littering the streets and civilians confined to cellars. According to the Ukrainian government, tens of thousands of people have been killed in the conflict.

Battle of Donbas

The war for the Donbas region, which comprises the provinces of Luhansk and Donetsk, might be decisive as Russia seeks a victory to justify President Vladimir Putin's invasion on Feb. 24.

Putin accuses Ukraine of mistreating Russian-speaking residents in the Donbas, an allegation that Kyiv and its Western allies dismiss as a fabrication used to legitimize an unjustified land grab.

On Wednesday, Russian media broadcast Putin speaking to a Luhansk child, saying, "It was the tragedy that occurred in the Donbas, including in the Lugansk People's Republic, that forced, simply forced Russia to launch this military operation, which everyone is well aware of today."

Peace negotiations have stopped. The Kremlin accused Kyiv of stalling and shifting its stance during the discussions. Kyiv accused Moscow of impeding talks by refusing humanitarian ceasefires, particularly those aimed at relieving the siege of Mariupol.

According to British military intelligence, fighting in the Donbas grew as Russian forces attempted to breach Ukraine's defenses, and Russia continued to build up troops on Ukraine's eastern border.

Moscow is anticipating that its superior weaponry will defeat Ukrainian defenses more effectively than during the unsuccessful campaign against Kyiv when nimble tiny forces attacked its overstretched supply lines.

On Tuesday, Russian forces captured Kreminna, a frontline town of 18,000 people, less than a day after commencing the Donbas attack. According to Ukraine's general staff, Russian forces attempted an offensive in Kharkiv, the country's second-largest city, which is located near Russia's supply routes to Donbas.

Within Kharkiv, where at least four people were killed in Tuesday's missile strikes, an older man's body lay face down outside a park on a suburban street, a ribbon of blood running into the gutter.

"He worked in security not far from here," Reuters was told by a resident named Maksym. "As soon as the bombardment began, everyone ran. Then we came out here and saw that the old man had already died."

Charles Michel, president of the European Council, representing the EU's 27 member states, arrived in Kyiv as the latest European leader to show his support.

Russian players will be banned from the Wimbledon tennis event, the latest proof of Russia's worldwide isolation, according to sports industry news site Sportico.

The All England Lawn Tennis Club, which hosts the grand slam, did not immediately respond to a request for comment. "To make sportspeople pawns of political intrigue is intolerable," Kremlin spokesman Dmitry Peskov stated.