Heavy fighting resumed in southern Ukraine's Mariupol on Saturday as Russian troops advanced in the center of the strategic Azov Sea port city.

In the besieged city, Ukrainian and Russian forces are fighting for the giant Azovstal steel plant, one of the biggest in Europe, said Vadym Denysenko, an adviser to Ukraine’s interior minister, in televised remarks on Saturday.

"Now there is a fight for Azovstal. ... I can say that we have lost this economic giant. In fact, one of the largest metallurgical plants in Europe is actually being destroyed," Denysenko said.

Late Friday, Ukrainian officials said that Mariupol lost its access to the Azov Sea, which is connected to the much larger Black Sea. Russia's Defense Ministry also said Friday that the army and its separatist allies had made a breakthrough in Mariupol and were now inside the city.

"In Mariupol, units of the Donetsk People's Republic, with the support of the Russian armed forces, are squeezing the encirclement and fighting against nationalists in the city center," the ministry said.

Russian media outlets also shared footage of troops engaging with Ukrainian holdouts in residential areas in the city.

The mayor of the city confirmed to the BBC that gunbattles had reached the heart of Mariupol, where rescuers were still searching for hundreds of people trapped under the wreckage of a bombed theater.

At the time of the attack, Mariupol's city council said that over 1,000 people were sheltering in the theater's basement when it was hit on Wednesday. On Friday, the council said one person had been badly wounded, but there were no dead, the only casualty tally given so far. There was still no information about potential fatalities, Ukrainian President Volodymyr Zelenskyy said, but 130 people had been saved so far and some were "heavily injured."

As the Russian offensive has met with fierce Ukrainian resistance, Moscow has increasingly turned to indiscriminate air and long-range strikes.

Meanwhile, Ukrainian authorities have not noticed any significant shifts over the past 24 hours in front line areas where Ukrainian troops are battling Russian forces, presidential adviser Oleksiy Arestovych said on Saturday.

In an online video address, he said fighting was ongoing and named Mariupol, the southern cities of Mykolaiv and Kherson, and the eastern town of Izyum as particular hot spots where Russian troops were on the offensive.

Ukrainian media reported that Russian forces had carried out a large-scale airstrike on Mykolaiv, killing at least 40 Ukrainian soldiers at their brigade headquarters.

Overnight, Zelenskyy accused the Kremlin of deliberately creating "a humanitarian catastrophe," but also appealed for Russian President Vladimir Putin to meet him for direct talks.

Zelenskyy said in his Friday nighttime video address to the nation that more than 9,000 people were able to leave Mariupol in the past day, and in all, more than 180,000 people have been able to flee through humanitarian corridors.

Ukraine hopes to evacuate civilians on Saturday via 10 humanitarian corridors from cities and towns on the front line of fighting with Russian forces, Deputy Prime Minister Iryna Vereshchuk also said.

She said a corridor had been agreed for the besieged city of Mariupol, although the authorities' previous efforts to evacuate civilians there under a temporary cease-fire have mostly failed, with both sides trading blame.

The United Nations migration agency says the fighting has displaced nearly 6.5 million people inside Ukraine, on top of the 3.2 million refugees who have already fled the country. Ukraine says thousands have been killed. The U.N. human rights office says that it has recorded a total of 816 civilians killed and 1,333 injured since the fighting began on Feb. 24, though it only reports counts that it can verify. It believes the figures vastly understate the actual toll. Ukrainian officials say thousands have been killed.

The war is now in its fourth week. The Russian military reported Saturday that it has used its latest hypersonic missile for the first time in combat. A spokesperson for the Russian Defense Ministry, Maj. Gen. Igor Konashenkov, said Kinzhal missiles destroyed an underground warehouse storing Ukrainian missiles and aviation ammunition in the western Ivano-Frankivsk region of Ukraine.

Shelling continued at the edges of Kyiv on Friday, while a barrage of missiles were launched against an aircraft repair installation at an airport outside the western city of Lviv, close to the border with Poland.

Ukraine said it had shot down two of six missiles launched in the attack from the Black Sea, which killed one person. The early morning strike was the closest one yet to the center of Lviv, which has become a crossroads for people fleeing from other parts of Ukraine and for others entering to deliver aid or join the fight.

The head of the Russian delegation in talks with Ukrainian officials said the parties have come closer to an agreement on a neutral status for Ukraine – one of the key Russian demands as its offensive continues. Vladimir Medinsky said Friday that the sides also have narrowed their differences on the issue of Ukraine dropping its bid to join NATO.

But Mikhailo Podolyak, an adviser to Zelenskyy, tweeted: "Our positions are unchanged. Cease-fire, withdrawal of troops & strong security guarantees with concrete formulas."

Russia's offensive remains largely stalled, a United States defense official said, with troops about 30 kilometers (20 miles) east of the capital Kyiv and facing heavy resistance. The official added that Russian forces had made no further progress into the northeastern city of Kharkiv, which they have encircled, and that Ukrainians were also defending the northern city of Chernihiv.

Britain's Defense Ministry said Russia was struggling to provide its forward troops "with even basic essentials such as food and fuel" because of Ukrainian attacks on their supply lines.