LVIV, Ukraine (AP) — Under relentless bombardment and a Russian blockade, the key port of Mariupol is holding out, but weapons and supplies shortages could weaken the resistance that has thwarted the Kremlin’s invasion plans.

More than six weeks after the Russian siege began, Ukrainian troops are continuing to fight the vastly superior Russian forces in ferocious battles amid the ruins of what once was a bustling city on the Sea of Azov.

The mayor says an estimated 120,000 people remain in the city, out of Mariupol’s prewar population of about 450,000.

The Ukrainians’ fight has scuttled Moscow’s designs, tying up significant Russian forces and delaying a planned offensive in eastern Ukraine’s industrial heartland, Donbas. The Kremlin hopes an attack in the east could reverse the battlefield fortunes for Russia after a humiliating failure to quickly storm the capital, Kyiv.

Mariupol has been a key objective for Russia since the start of the Feb. 24 invasion. Capturing the city would allow Moscow to establish a land corridor to Ukraine’s Crimean Peninsula that Russia annexed in 2014 and deprive Ukraine of a major port and prized industrial assets.

Oleksandr Motuzyanyk, a spokesman for Ukraine’s Ministry of Defense, described the situation in Mariupol as “complicated,” saying fighting is continuing in industrial areas and the port, and that Russia for the first time used a Tu-22?3 long-range bomber to attack the city.

The giant Azovstal steel mill and other plants have been heavily damaged by the Russian bombardment that has flattened much of Mariupol, indiscriminately hitting homes, hospitals and other public buildings and killing thousands.

The victims include about 300 people killed in last month’s Russian airstrike on the Mariupol Drama Theater that was being used as a shelter and had the word “CHILDREN” printed in Russian in huge white letters on the pavement outside to ward off aerial attack.

Mayor Vadym Boychenko told The Associated Press that at least 21,000 people were killed in Mariupol with bodies “carpeted through the streets.” He said the Russians deployed mobile cremation equipment to methodically dispose of the bodies in order to hide evidence of the massacre and prevent international organizations from documenting “the horror the Russian army is responsible for.”

The bodies of more than 900 civilians have been found in the region surrounding Kyiv following the withdrawal of Russian forces, said Andriy Nebytov, head of the regional police force, adding that many were “simply executed.” The number of dead is double what was announced nearly two weeks ago, a discovery that has fueled global outrage and accusations from Ukrainians and the West that Russia is committing war crimes in Ukraine.

Moscow deployed fighters from Chechnya, known for their ferocity, to wage street battles in Mariupol. Chechnya’s Moscow-backed leader, Ramzan Kadyrov, has repeatedly boasted on his messaging app channel about defeating Ukrainians in Mariupol, but the fight has continued.

Boychenko said several Ukrainian units are still fighting in Mariupol, including the 36th Marine Brigade, Interior Ministry troops, border guards and the national guard’s Azov Regiment, which Russia singles out as a particular villain because of its far-right ideology.

The Azov Regiment, a seasoned volunteer force that is widely considered one of the country’s most capable units, is defending the Azovstal plant that covers an area of nearly 11 square kilometers (over 4.2 square miles). It has taken advantage of the plant’s sprawling network of concrete buildings and underground facilities to repel continuous Russian attacks.

The 36th Marine Brigade was maintaining defensive positions at the Azovmash and Zavod Ilyicha factories until it ran out of supplies and ammunition and made a desperate attempt to break through the Russian blockade earlier this week.

In a post on the brigade’s Facebook page, one of its officers described how “for more than a month, the marines have been fighting without replenishing ammunition, food and water supplies.”

“The wounded accounted for nearly a half of the brigade’s strength, but those who still had their limbs and were capable of walking reported back to duty,” it said.

Boychenko said that some of the marines managed to join the Azov regiment, while others were captured by the Russians. He didn’t give any numbers.

The Russian military said Thursday that a total of 1,160 Ukrainian marines surrendered this week, a claim that couldn’t be independently verified.

As the Ukrainian troops continue to offer fierce resistance in Mariupol, fears have grown that the exasperated Russians could resort to chemical weapons to deal with the remaining pockets of resistance at the Azovstal plant and other areas of the city.

Eduard Basurin, a Russia-allied separatist official in eastern Ukraine, appeared to call for that Monday, telling Russian state TV that the Russia-backed forces should block all the exits out of the factory and then “use chemical troops to smoke them out of there.” He later said that no chemical weapons were used.

The Azov Regiment claimed Monday, without providing evidence, that a drone had dropped a poisonous substance on its positions but inflicted no serious injuries. A Ukrainian defense official said the attack possibly involved phosphorus munitions.

Ukrainian authorities have said that the Russians have blocked humanitarian convoys from reaching Mariupol, keeping it without food, water and power since the siege started. The Russian troops have turned back buses sent to evacuate residents, but about 150,000 have been able to flee the city in their own vehicles.

Boychenko said at least 33,500, and, possibly, up to 50,000 Mariupol residents have been taken to “filtration camps” in the separatist-controlled east before being forcibly sent to distant, economically depressed areas in Russia.

Mariupol has seen communications cut since the start of the siege, and as the Russians moved to capture sections of the city they launched radio broadcasts to brainwash the population.

“They unleashed propaganda, telling people that Kyiv and other cities have been captured and they have been abandoned,” Boychenko said.

The continuing fighting has forced the Russian military to keep a significant number of troops in the city, delaying the eastern offensive.

“As long as the street fighting is going on, Russia can’t remove troops from Mariupol and deploy them to other areas, including Donbas,” Oleh Zhdanov, an independent military expert, told the AP.

“The Ukrainian troops in Mariupol are still fulfilling their main task by diverting the Russian forces from other areas. Mariupol remains a major symbol of the Ukrainian resistance.”

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