KYIV, UKRAINE -- Russian forces destroyed a theatre in Mariupol where hundreds of people were sheltering Wednesday and rained fire on other cities, Ukrainian authorities said, even as the two sides projected optimism over efforts to negotiate an end to the fighting.

The airstrike ripped apart the centre of the once-elegant building, where hundreds of civilians had been living since their homes had been destroyed in the fighting, Ukraine's foreign ministry said in a statement.

Many people were buried in the rubble, the statement said, though there was no immediate word on how many had been killed or injured. Satellite imagery from Monday showed the word "CHILDREN" written in Russian in large, white capital letters on the pavement in front of and behind the building, the Maxar space technology company said.

"My heart breaks from what Russia is doing to our people," Ukrainian President Volodymyr Zelensky said in his nightly address, calling for more sanctions on Russia after the bombing.

The Russian defence ministry denied bombing the theatre or anywhere else in Mariupol on Wednesday.

In Kyiv, residents huddled in homes and shelters during a citywide curfew that was set to run until Thursday morning, as Russian troops shelled areas in and around the city, including a residential neighbourhood 2.5 kilometres (1.5 miles) from the presidential palace. A 12-story apartment building in central Kyiv erupted in flames after being hit by shrapnel.

And 10 people were killed while standing in line for bread in the northern city of Chernihiv, the Ukrainian General Prosecutor's Office said.

Earlier Wednesday, Zelensky went before the U.S. Congress via video and, invoking Pearl Harbor and 9/11, pleaded with America for more weapons and tougher sanctions against Russia, saying: " We need you right now."

U.S. President Joe Biden announced that the U.S. is sending an additional $800 million in military aid to Ukraine, including more anti-aircraft and anti-tank weapons and drones. He also called Vladimir Putin a "war criminal" in his sharpest condemnation of the Russian leader since the invasion began.

International pressure against the Kremlin mounted and its isolation deepened as the International Court of Justice, also known as the World Court, ordered Russia to stop attacking Ukraine, though there was little hope it would comply. Also, the 47-nation Council of Europe, the continent's foremost human rights body, expelled Russia.

While Moscow's ground advance on the Ukrainian capital appeared largely stalled, Putin said the operation was unfolding "successfully, in strict accordance with pre-approved plans." He also decried Western sanctions against Moscow, accusing the West of trying to "squeeze us, to put pressure on us, to turn us into a weak, dependent country."

A senior U.S. defence official said the Russians were still making little tangible progress in much of the country, but have begun shelling the suburbs of Odesa, Ukraine's third-largest city and a major naval and shipping hub. The official, who spoke on condition of anonymity to discuss U.S. military assessments, said Russia's aims weren't clear, but Western officials have long worried about a ground assault on the coastal city.

The two sides resumed talks via video on Wednesday, with Zelensky adviser Mikhailo Podolyak saying Ukraine was demanding a cease-fire, the withdrawal of Russian troops and legal security guarantees for Ukraine from several countries.

"This is possible only through direct dialogue" between Zelensky and Putin, he tweeted.

An official in Zelensky's office told The Associated Press that the main subject under discussion was whether Russian troops would remain in the two separatist regions of eastern Ukraine after the war and where the borders would be.

Just before the war, Russia recognized the independence of two regions controlled by Russian-backed separatists since 2014 and extended the borders of those regions to areas Ukraine had continued to hold, including the strategically important port city of Mariupol, which has endured a brutal siege.

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 the signing of a legally binding document with security guarantees for Ukraine. In exchange, the official said, Ukraine was ready to discuss a neutral status.

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

After Tuesday's negotiations, Russian Foreign Minister Sergey Lavrov said a neutral military status for Ukraine was being "seriously discussed" by the two sides, while Zelensky said Russia's demands for ending the war were becoming "more realistic."

Hopes for diplomatic progress to end the war rose after Zelensky acknowledged Tuesday in the most explicit terms yet that Ukraine is unlikely to realize its goal of joining NATO. Putin has long depicted Ukraine's NATO aspirations as a threat to Russia.

Lavrov welcomed Zelensky's comment and said "the businesslike spirit" starting to surface in the talks "gives hope that we can agree on this issue."

"A neutral status is being seriously discussed in connection with security guarantees," Lavrov said on Russian TV. "There are concrete formulations that in my view are close to being agreed."

Prospects for a diplomatic breakthrough were highly uncertain, however, given the gulf between Ukraine's demand that the invading forces withdraw completely and Russia's suspected aim of replacing Kyiv's Westward-looking government with a pro-Moscow regime.

The fighting has led more than 3 million people to flee Ukraine, by the United Nations' estimate. The overall death toll remains unknown, though Ukraine has said thousands of civilians have died.

Speaking to Congress, Zelensky said Russia "has turned the Ukrainian sky into a source of death." But Biden has rejected Zelensky's requests to send warplanes to Ukraine or establish a no-fly zone over the country because of the risk of triggering war between the U.S. and Russia.

Amid the vast humanitarian crisis caused by the war, the Red Cross has helped evacuate civilians from besieged areas and has delivered 200 tons of aid, including medical supplies, blankets, water and over 5,200 body bags to help "ensure the dead are treated in a dignified manner."

Nowhere has suffered more than Mariupol, where local officials say missile strikes and shelling have killed more than 2,300 people. The southern seaport of 430,000 has been under attack for almost all of the three-week war in a siege that has left people struggling for food, water, heat and medicine.

Local authorities said Russian forces took hundreds of people hostage at a Mariupol hospital and were using them as human shields.

Using the flashlight on his cellphone to illuminate a hospital basement, Dr. Valeriy Drengar pulled back a blanket to show the body of an infant 22 days old. Other wrapped bodies also appeared to be children, given their size.

"These are the people we could not save," Drengar said.

Nearly 30,000 people managed to escape the city Tuesday in thousands of vehicles by way of a humanitarian corridor, city officials said. Zelensky said 6,000 more left on Wednesday, including 2,000 children, but evacuations elsewhere were stopped because of Russian shelling.

Kyiv regional leader Oleksiy Kuleba said Russian forces had intensified fighting in the Kyiv suburbs and a highway leading west, and across the capital region, "kindergartens, museums, churches, residential blocks and engineering infrastructure are suffering from the endless firing."

In other developments, the mayor of the city of Melitopol, who was seized by Russian forces five days ago, has been freed, said Zelensky chief of staff Andriy Yermak. No details were given about how he became free.

Ukraine also appeared to have successes, with satellite photos from Planet Labs PBC analyzed by the AP showing helicopters and vehicles ablaze at the Russian-held Kherson airport and air base after a suspected Ukrainian strike on Tuesday.

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Associated Press writers Yuras Karmanau, in Lviv, Ukraine, Lolita C. Baldor in Washington and other AP journalists around the world contributed to this report.

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