LVIV, Ukraine (AP) -- Russian forces pressed their assault on Ukrainian cities Friday, with new missile strikes and shelling on the edges of the capital Kyiv and the western city of Lviv, as world leaders pushed for an investigation of the Kremlin's repeated attacks on civilian targets, including schools, hospitals and residential areas.

Outside Lviv, black smoke billowed for hours after the early morning strike by several missiles, which the mayor said hit a facility for repairing military aircraft near the city's international airport, also damaging a bus repair facility. No casualties were immediately reported. The facility had suspended work ahead of the attack, said the mayor, Andriy Sadovyi, on the Telegram messaging app.

A soldier standing guard near the site said he heard three blasts in quick succession around 6 a.m. A nearby resident described his building vibrating from the explosions and people panicking.

The missiles that hit Lviv were launched from the Black Sea, but two of the six that were launched were shot down, Ukrainian air force's western command said on Facebook.

Not far from the Polish border and well behind the front lines, Lviv and the surrounding area have not been spared Russia's attacks. In the worst, nearly three dozen people were killed last weekend in a strike on a training facility near the city. Meanwhile, Lviv's population has swelled by some 200,000 as people from elsewhere in Ukraine have sought shelter there.

Early morning barrages also hit on the northern edges of Kyiv. At least one person was killed by shelling on Podil, a neighborhood just north of downtown Kyiv, according to emergency services. It was not immediately known what was hit in the bombardment.

In city after city around Ukraine, hospitals, schools and buildings where people sought safety from the bombardment have been attacked. Rescue workers searched for survivors in the ruins of a theater that served as a shelter when it was blown apart by a Russian airstrike in the besieged southern city of Mariupol. And in Merefa, near the northeast city of Kharkiv, at least 21 people were killed when Russian artillery destroyed a school and a community center, a local official said.

In the northern city of Chernihiv, dozens of bodies were brought to the morgue in just one day.

U.S. Secretary of State Antony Blinken said Thursday that American officials were evaluating potential war crimes and that if the intentional targeting of civilians by Russia is confirmed, there will be "massive consequences."

The United Nations political chief, Undersecretary-General Rosemary DiCarlo, also called for an investigation into civilian casualties, reminding the UN Security Council that international humanitarian law bans direct attacks on civilians.

She said many of the daily attacks battering Ukrainian cities "are reportedly indiscriminate" and involve the use of "explosive weapons with a wide impact area." DiCarlo said the devastation in Mariupol and Kharkiv "raises grave fears about the fate of millions of residents of Kyiv and other cities facing intensifying attacks."

In Mariupol, hundreds of civilians were said to have taken shelter in a grand, columned theater in the city's center when it was hit Wednesday by a Russian airstrike. More than a day later, there were no reports of deaths and conflicting reports on whether anyone had emerged from the rubble. Communications are disrupted across the city and movement is difficult because of shelling and other fighting.

Satellite imagery on Monday from Maxar Technologies showed huge white letters on the pavement outside the theater spelling out "CHILDREN" in Russian -- "DETI" -- to alert warplanes to the vulnerable people hiding inside."We hope and we think that some people who stayed in the shelter under the theater could survive," Petro Andrushchenko, an official with the mayor's office, told The Associated Press. He said the building had a relatively modern basement bomb shelter designed to withstand airstrikes. Other officials said earlier that some people had gotten out.

Video and photos provided by the Ukrainian military showed that the at least three-story building had been reduced to a roofless shell, with some exterior walls collapsed.

Across the city, snow flurries fell around the skeletons of burned, windowless and shrapnel-scarred apartment buildings as smoke rose above the skyline.

"We are trying to survive somehow," said one Mariupol resident, who gave only her first name, Elena. "My child is hungry. I don't know what to give him to eat."

She had been trying to call her mother, who was in a town 50 miles (80 kilometers) away. "I can't tell her I am alive, you understand. There is no connection, just nothing," she said.

Cars, some with the "Z" symbol of the Russian invasion force in their windows, drove past stacks of ammunition boxes and artillery shells in a neighborhood controlled by Russian-backed separatists.

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

In Chernihiv, at least 53 people were brought to morgues over 24 hours, killed amid heavy Russian air attacks and ground fire, the local governor, Viacheslav Chaus, told Ukrainian TV on Thursday.

Ukraine's emergency services said a mother, father and three of their children, including 3-year-old twins, were killed when a Chernihiv hostel was shelled. Civilians were hiding in basements and shelters across the embattled city of 280,000.

"The city has never known such nightmarish, colossal losses and destruction," Chaus said.

The World Health Organization said it has verified 43 attacks on hospitals and health facilities, with 12 people killed and 34 injured.

In remarks early Friday, Ukrainian President Volodymyr Zelenskyy said he was thankful to President Joe Biden for additional military aid, but he would not get into specifics about the new package, saying he did not want Russia to know what to expect. He said when the invasion began on Feb. 24, Russia expected to find Ukraine much as it did in 2014, when Russia seized Crimea without a fight and backed separatists as they took control of the eastern Donbas region.

Instead, he said, Ukraine had much stronger defenses than expected, and Russia "didn't know what we had for defense or how we prepared to meet the blow."

In a joint statement, the foreign ministers of the Group of Seven leading economies accused Putin of conducting an "unprovoked and shameful war," and called on Russia to comply with the International Court of Justice's order to stop its attack and withdraw its forces.

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.

Zelenskyy said he would not reveal Ukraine's negotiating tactics.

"Working more in silence than on television, radio or on Facebook," Zelenskyy said. "I consider it the right way."

While details of Thursday's talks were unknown, an official in Zelenskyy's office told the AP that on Wednesday, the main subject discussed 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 legally binding 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.