KYIV, Ukraine (NewsNation) —  The long battered city of Mariupol appeared on the brink of falling to Russian forces Sunday after eight weeks under siege, a new development that would give Moscow crucial success in Ukraine after failed attempts to capture key cities.

The Russian military estimated that about 2,500 Ukrainian fighters are holding out at a hulking steel plant with a warren of underground passageways provided the last pocket of resistance in Mariupol. Russia gave another deadline for their surrender, saying those who put down their weapons were “guaranteed to keep their lives,” but Ukraine continued to fight back.

“All those who will continue resistance will be destroyed,” Maj. Gen. Igor Konashenkov, the Russian Defense Ministry’s spokesman, said. He said intercepted communications indicated there were about 400 foreign mercenaries along with the Ukrainian troops at the Azovstal steel mill, a claim that couldn’t be independently verified.

Seizing Mariupol would free up Russian troops to weaken and enclose Ukrainian forces in eastern Ukraine, where Russia has focused its war aims and is deploying personnel and equipment withdrawn from the north after the failure to take Kyiv.

Ukrainian Deputy Defense Minister Hanna Malyar described Mariupol as a “shield defending Ukraine” as Russian troops prepare for a full-scale offensive in Donbas, the country’s eastern industrial heartland where Moscow-backed separatists already control some territory.

After losing the flagship of its Black Sea Fleet, Russia’s military command vowed Friday to step up missile strikes on the capital. The Russian military said Sunday that it had attacked an ammunition plant near Kyiv overnight with precision-guided missiles, the third such strike in as many days.

Russia’s stepped-up attacks on Kyiv came after it accused Ukraine on Thursday of wounding seven people and damaging about 100 residential buildings with airstrikes in Bryansk, a region bordering Ukraine. Ukrainian officials have not confirmed hitting targets in Russia.

Kyiv Mayor Vitali Klitschko said a Saturday strike on an armored vehicle plant killed one person and wounded several. He advised residents who fled the city earlier in the war not to return.

The Russian military also claimed Sunday to have destroyed Ukrainian air defense radars in the east, near Sievierodonetsk, as well as several ammunition depots elsewhere. Explosions were reported overnight in Kramatorsk, an eastern city where rockets killed at least 57 people at a train station crowded with civilians trying to evacuate before the expected Russian offensive.

The ongoing siege and relentless shelling of Mariupol have come at a terrible cost, with officials estimating Russians had killed at least 21,000 people. About 120,000 people remain in the city, out of a prewar population of 450,000.

Malyar, the deputy defense minister, said the Russians have continued to hit Mariupol with airstrikes and could be getting ready for an amphibious landing to beef up their ground forces. Capturing the city would also allow Russia to secure a land corridor to the Crimean Peninsula, which it seized from Ukraine in 2014.

So far, tunnels at the sprawling Azovstal steel mill, which covers an area of more than 4.2 square miles, have allowed the defenders to hide and resist until they run out of ammunition.

With Russia apparently poised to declare victory, Ukrainian President Volodymyr Zelenskyy said the city’s fall could scuttle any attempt at a negotiated peace.

“The destruction of all our guys in Mariupol — what they are doing now — can put an end to any format of negotiations,” Zelenskyy said in an interview with Ukrainian journalists.

In his overnight address, Zelenskyy called on the West to send more heavy weapons immediately if there is any chance of saving the city, adding Russia “is deliberately trying to destroy everyone who is there.”

Austrian Chancellor Karl Nehammer, who met with Vladimir Putin this past week in Moscow, the first European leader to do so since the invasion began Feb. 24, said the Russian president is “in his own war logic” on Ukraine.

In an interview on NBC’s “Meet the Press,” Nehammer said he thinks Putin believes he is winning the war and “we have to look in his eyes and we have to confront him with that, what we see in Ukraine.’’

Nehammer said he told Putin what he saw during a visit to the Kyiv suburb of Bucha, where more than 350 bodies have been found along with evidence of killings and torture under Russian occupation, and “it was not a friendly conversation.”

Italian Premier Mario Draghi called Ukraine’s continued resistance to Russia’s invasion “heroic,” depriving Russia of what it had expected to be a speedy victory.

“What awaits us is a war of resistance, prolonged violence with destruction that will continue,” Draghi told Italian daily newspaper Corriere della Sera in an interview published Sunday. “There is no sign that the Ukraine population can accept a Russian occupation.”

Like Mariupol, the northeast city of Kharkiv has been an ongoing target of Russian aggression. Mayor Ihor Terekhov said three people were killed and 34 wounded Saturday.

One explosion believed to have been caused by a missile sent rescue workers scrambling near an outdoor market. The workers said one person was killed and at least 18 wounded. Local officials said 10 people died in rocket attacks striking residential areas of Kharkiv on Friday.

Nate Mook, a member of the World Central Kitchen NGO run by celebrity chef José Andrés, said in a tweet that four workers in Kharkiv were wounded by a strike. Andrés tweeted that staff members were unnerved but safe.

Zelenskyy estimated that 2,500 to 3,000 Ukrainian troops have died in the war, and about 10,000 have been wounded. The office of Ukraine’s prosecutor general said Saturday that at least 200 children have been killed, and more than 360 wounded.

Even as fighting continues, Zelenskyy said he’s planned a memorial to honor the dead and the sacrifices of the Ukrainian people.