KIEV, Ukraine — Rescuers searched for survivors Thursday in the ruins of a theater ripped apart by a Russian airstrike in the besieged city of Mariupol, while a ferocious bombardment left dozens dead in a northern city over the past day, authorities said.

Hundreds of civilians had been living in the grand, columned theater in central Mariupol after their homes were destroyed in three weeks of fighting in the strategic port city.

Nearly a day after the attack, there were no reports of deaths. With intense street fighting cutting off much of the city from the flow of information, there were also conflicting reports about whether survivors had emerged from the rubble.

“We hope and we think that some people who stayed in the shelter under the theater could survive,” an official with the mayor of Mariupol’s office, Petro Andrushchenko, told the Associated Press. He said the building had a relatively modern basement bomb shelter designed to withstand airstrikes.

Other officials had said earlier that some people had escaped the destruction. Ukraine’s ombudswoman, Ludmyla Denisova, said on the Telegram messaging app that the shelter had not collapsed in the bombing. She and parliament member Sergiy Taruta said some people had survived.

At least as recently as Monday, huge white letters on the ground in front of and behind the theater spelled out “CHILDREN” in Russian — “DETI” — to alert warplanes of those inside, according to images released by the Maxar space technology company.

Russia’s military denied bombing the theater or anyplace else in Mariupol on Wednesday.

The strike against the theater was part of a furious bombardment of civilian targets in multiple cities over the past few days.

A municipal pool complex where pregnant women and women with children were taking shelter was also hit Wednesday, according to head of the Donetsk regional administration, Pavlo Kyrylenko. There was no word on casualties in that strike.

To the north, at least 53 people were brought to morgues over the past 24 hours in the city of Chernihiv, killed amid heavy Russian airstrikes, artillery bombardment, and ground fire, the local governor, Viacheslav Chaus, told Ukrainian TV on Thursday. Russia denied involvement.

Civilians hid in basements and shelters in the city of 280,000

“The city has never known such nightmarish, colossal losses and destruction,” Mr. Chaus said.

Chernihiv, which is near the borders of Belarus and Russia, was among the first Ukrainian cities to come under attack when Russia invaded.

President Zelensky called for more help for his country in a video address to German lawmakers, saying thousands of people have been killed, including 108 children. He also referred to the dire situation in Mariupol, saying: “Everything is a target for them.”

Mr. Zelensky’s office said Russia carried out further airstrikes on Mariupol early Thursday and attacks elsewhere around the country overnight, including in the Kalynivka and Brovary suburbs of the capital, Kiev. There was no immediate word on casualties.

In Kiev, where residents have been huddling in homes and shelters, emergency authorities said a fire broke out in an apartment building hit by remnants of a downed Russian rocket, killing one person. Firefighters evacuated 30 people from the top floors of the 16-story building and put out the blaze.

Russian artillery destroyed a school and a community center in Merefa, near the northeast city of Kharkiv, according to the mayor of Merefa, Veniamin Sitov. There were no known civilian casualties. The Kharkiv region has seen heavy bombardment as stalled Russian forces try to advance in the area.

The UN Security Council planned to meet Thursday.

“Russia is committing war crimes and targeting civilians,” Britain’s UN delegation tweeted. “Russia’s illegal war on Ukraine is a threat to us all.”

President Vladimir Putin went on television Wednesday to excoriate Russians who don’t back him.

Russians “will always be able to distinguish true patriots from scum and traitors and will simply spit them out like a gnat that accidentally flew into their mouths,” said Mr. Putin, using language reminiscent of the Stalin era. “I am convinced that such a natural and necessary self-purification of society will only strengthen our country.”

He said the West is using a “fifth column” of traitorous Russians to create civil unrest. “And there is only one goal, I have already spoken about it — the destruction of Russia,” he said.

The speech appeared to be a warning that his authoritarian rule, which had already grown tighter since the invasion began on February 24, shutting down Russian news outlets and arresting protesters, could grow even more repressive.

In a sign of that, Russian law enforcement announced the first known criminal cases under a new law that allows for 15-year prison terms for posting what is deemed to be “false information” about the Ukraine war. Among those charged was a Russian-language cookbook author and blogger living abroad, Veronika Belotserkovskaya.

Both Ukraine and Russia this week reported some progress in negotiations. Kremlin spokesman Dmitry Peskov said Thursday that some negotiators were breaking into working groups, “but there should be contacts today.”

Talks were held by video Wednesday. An official in Mr. Zelensky’s office told the AP that the main subject under discussion was whether Russian troops would remain in separatist regions in eastern Ukraine after the war and where the borders would be.

The official, speaking on condition of anonymity to discuss the sensitive talks, said Ukraine was insisting on the inclusion of one or more Western nuclear powers in the negotiations and on a legally binding document with security guarantees for Ukraine.

In exchange, the official said, Ukraine was ready to discuss a neutral military status.

Russia has demanded that NATO pledge never to admit Ukraine to the alliance or station forces there.

The fighting has led more than 3 million people to flee Ukraine, the UN estimates. The death toll remains unknown, though Ukraine has said thousands of civilians have died.