MUKACHEVO, Ukraine - Russia is renewing its attacks on the last Ukrainian holdouts in the besieged port city of Mariupol, an adviser to Ukrainian President Volodymyr Zelensky said Saturday, accusing Russia of a surge in airstrikes and alleging preparations to storm the Azovstal Iron and Steel Works, where many of the city’s remaining defenders are based.

“The enemy is trying to suppress the last resistance of the defenders of Mariupol,” Oleksiy Arestovych said in a video posted to Telegram. “Our defenders are withstanding it, despite a very difficult situation, and are even carrying out counter maneuvers.” Russian President Vladimir Putin publicly ordered his forces Thursday not to storm the site but to seal it off.

Arestovich said Saturday that Russian troops were trying to storm the plant but were met with “counter maneuvers” from forces at the sprawling facility. The Washington Post could not independently verify the claim. “The enemy is trying to suppress the last resistance of the defenders of Mariupol and in the region of Azovstal has resumed airstrikes . . . and attempts to carry out storming activities,” he said in a video on Telegram.

Putin said Thursday that he had ordered his troops not to storm the steel plant but to blockade it “so that even a fly could not get through.” The Russian leader declared victory in an offensive to control the strategic coastal city although Ukrainian fighters holed up there rejected Moscow’s deadlines to surrender.

A video released Saturday by Ukrainian forces at their last stronghold in the Azovstal steel plant in the southern port of Mariupol appears to show a large number of civilians living in cramped conditions in an underground bunker, the majority appearing to be women and children.

The video, if confirmed, would be the most extensive footage to date of life in the Azovstal Iron and Steel Works, where an unspecified number of Ukrainian civilians and fighters - members of the 36th Separate Marine Brigade and the Azov Regiment - are said to be holding out against a much larger and better-equipped Russian force. The video could not be independently verified.

In the video, members of the Azov Regiment are shown descending a flight of stairs - past graffiti on the walls in large letters in Russian saying “children” - to deliver food to civilians in an underground basement with a steel door.

A large group of women and children gathers to receive the delivery. None are identified. The shelter appears to be home to a few dozen people, tightly crowded and with their belongings suspended on lines above their beds to maximize the use of the space.

Ukrainian commanders, speaking previously to The Washington Post over a satellite connection, said that beneath the plant is a system of underground tunnels and rooms similar to the one shown in the video, where they find refuge from the shelling. It is not clear how many civilian shelters exist beneath the factory complex.

The commanders say there are “hundreds” of civilians. Ukrainian government officials have said there could be as many as 1,000.

The Azov Regiment’s deputy commander, Sviatoslav Palamar, told the Associated Press that the video was recorded Thursday.

Russia has said the Ukrainians can leave the plant if they lay down their arms. The Ukrainian soldiers have refused, saying they do not trust the Russian forces and have insisted on an evacuation and guarantee of safety by a third country. In the video shared Saturday, some civilians said they wanted to be evacuated to Ukrainian-controlled territory and demanded a halt to all fighting.

The civilians also said they were running out of food. Some said they had been there since the end of February or the beginning of March and moved to the plant because it seemed at that time a refuge from the shelling.

The Azov Regiment is a nationalist group that is part of Ukraine’s National Guard and has been a key component of Ukraine’s defense of Mariupol. In the past, the group has previously been connected to the extreme right, but during Ukraine’s war against Russia, it has become a magnet for fighters from a wide array of backgrounds and political persuasions, or with no political affiliations.

The renewed focus on Mariupol came after Zelensky warned late Friday that Russia poses a threat to more countries in the region, cautioning that the invasion of Ukraine was “intended only as a beginning” and that the Russians “want to capture other countries.” Zelensky made the remark after a Russian commander said the Kremlin intends to establish a path through Ukraine to a breakaway territory in Moldova. Moscow declined to confirm whether this was official policy; some analysts said they doubt that Russia has the capability.

A flurry of diplomatic endeavors to end the war continues, even as Russia claims that talks with Ukraine have stalled. U.N. Secretary General António Gutierrez heads to Moscow on Tuesday to meet with Putin, before visiting Ukraine on Thursday for discussions with Zelensky. Turkish President Recep Tayyip Erdogan, who has hosted a previous round of peace talks, plans to call Putin and Zelensky in the coming days to push for a leaders’ summit in Istanbul.

Zelensky said that life in some parts of Ukraine that have been liberated from Russian forces is starting to return to normal, nearly two months into Moscow’s invasion.

Zelensky, who warned late Friday that Russia poses a threat to more countries in the region and that the invasion of Ukraine was “intended only as a beginning,” said he was grateful to nations that have supplied his country with weapons that “will be able to save the lives of thousands of people.”

“And we will be able to show the occupiers that the day when they will be forced to leave Ukraine is approaching,” Zelensky said in his address. “The return to normal life in the territories liberated from the occupiers continues.”

The Ukrainian president said the return of some normality in Ukraine’s cities and communities signifies “the return of life in the full sense of these words.”

“I believe that such a return will take place in the south of our country and in the east of Ukraine,” he said in his address, “in all areas where degradation, destruction and death have been brought under the Russian flag.”

He concluded his speech by stressing to Ukrainians that a return to normal life means not cooperating with Russian forces.

“Ignore the occupiers,” he said. “Do not cooperate with them. Don’t help them.”

In other developments, a missile struck what appeared to be a residential area of Odessa, a port city on the Black Sea, authorities said Saturday.

The city’s official Telegram account said infrastructure was hit and asked residents not to share photos or videos, on the grounds that doing so would “help the enemy.” No information was immediately available about what specifically was struck and whether anyone was killed.

“The only aim of Russian missile strikes on Odesa is terror,” tweeted Dmytro Kuleba, Ukraine’s foreign minister. “Russia must be designated a state sponsor of terrorism and treated accordingly. No business, no contacts, no cultural projects. We need a wall between civilization and barbarians striking peaceful cities with missiles.”

As an economically important port city, Odessa has been a target for Russian forces. But Moscow’s military has struggled to advance past Mykolaiv to the east, and Odessa has been attacked much less than many other cities.

Life had largely returned to normal there, and many residents are preparing to celebrate Orthodox Easter on Sunday. The city has implemented a curfew from 11 p.m. Saturday to 5 a.m. Sunday because the holiday weekend is considered at higher risk for attacks.

The governor of the embattled Luhansk region in Ukraine’s east said Saturday that the area has been under heavy Russian shelling, as other areas experienced a worsening humanitarian crisis, including water shortages.

Serhiy Haidai said on Telegram that the cities of Severodonetsk and Lysychansk were without water and that some 5,000 residences there had no electricity. He accused Russian forces of hitting power lines as well as substations that pump water to homes. Emergency crews were working to restore both amid heavy shelling, he said. The cities of Popasna and Rubizhne also were facing water shortages.

Earlier Saturday, Haidai said in a video that Ukrainian-controlled cities in Luhansk were being shelled. Moscow is intensifying its military campaign in Ukraine’s east, Russian Foreign Minister Sergei Lavrov said this week. Russia seeks the “complete liberation” of the Luhansk and Donetsk regions, including besieged Mariupol, he said. Earlier this week, Haidai said Russian forces were in control of 80 percent of Luhansk, which is part of the broader eastern Donbas region of Ukraine.

Urging civilians to flee, Haidai said that some evacuations from Severodonetsk were conducted Saturday and that trains would run from the station in Pokrovsk to evacuate residents from the Luhansk and Donetsk regions. Two people were killed by shelling in Popasna and street fighting was underway there, he said. The Washington Post could not independently verify his claims.