Russia's invasion of Ukraine entered its fourth week on Thursday, with Russian forces largely bogged down outside major cities and shelling them from a distance, raining havoc on civilians.

A Russian airstrike just before dawn Thursday killed 21 people and destroyed a school and community center in Merefa, near the northeast city of Kharkiv, officials said.

The governor of the northern city of Chernihiv said it has experienced "colossal losses and destruction" from Russian artillery and air strikes. Viacheslav Chaus told Ukrainian TV on Thursday that the bodies of 53 people had arrived at city morgues over the past 24 hours. A hostel was shelled, killing a mother, father and three of their children, including 3-year-old twins, officials said.

In the besieged southern city of Mariupol, there was an ongoing search for survivors after a Russian airstrike Wednesday hit a theater where hundreds of people were sheltering, local officials said. The building reportedly withstood the impact but the entrance was blocked by rubble. Although officials said some people had escaped, the number of casualties remained unknown.

During a video address to German lawmakers, Ukrainian President Volodymyr Zelenskyy condemned Russia for the dire situation in Mariupol, saying: "Everything is a target for them."

Foreign ministers of the Group of Seven leading economies said in a joint statement that Russian President Vladimir Putin was conducting an "unprovoked and shameful war." Putin, meanwhile, lashed out at "scum and traitors," signaling an even harsher crackdown on domestic opposition to the war.

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

Here are some key things to know about the conflict:

WHAT IS HAPPENING ON THE GROUND?

"Several hundred" residents were sheltering in Mariupol's Drama Theater when it was hit Wednesday, Pavlo Kyrylenko, the head of the Donetsk regional administration, wrote on Telegram. He said the airstrike also hit the Neptune swimming pool complex, burying women and children.

Satellite imagery from the Maxar space technology firm showed the word "CHILDREN" written in large white letters in Russian outside the theater building as recently as Monday.

The Russian Defense Ministry denied bombing any targets in Mariupol on Wednesday.

A fire broke out in an apartment building in the capital, Kyiv, early on Thursday after it was hit by the remnants of a downed Russian rocket, killing one person and injuring at least three others, according to emergency services.

Fighting continued in Kyiv's suburbs, depriving thousands of heat and clean water.

In Chernihiv, the governor said civilians were hiding in basements and shelters without access to utilities in the city of 280,000 people. Close to the borders with Belarus and Russia, Chernihiv was among the first Ukrainian cities to come under attack when the invasion began on Feb. 24.

Britain's Defense Ministry said Russia's invasion "has largely stalled on all fronts" amid stiff Ukrainian resistance. It said Russian forces have made "minimal progress" on land, sea or air in recent days, and are suffering heavy losses.

Ukrainian forces are using inexpensive Turkish-made drones to carry out lethal attacks on the Russian invaders.

WHAT HAS THE AP DIRECTLY WITNESSED OR CONFIRMED?

The attacks in Mariupol knocked out the windows of apartment buildings and sent smoke rising above the skyline.

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.

One resident, who only gave her first name of Elena, said 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.

Workers in the city have braved the relentless shelling to dump the bodies of children in a mass grave. Local officials have tallied 2,500 deaths in the siege, but many bodies crushed in the rubble can't be counted because of the assault.

In Kharkiv, doctors are struggling to treat COVID-19 patients as the bombs fall outside. Several times a day, air raid sirens wail at a local hospital, sending virus patients — some connected to ventilators and struggling to breathe — into bomb shelters.

"Bombing takes place from morning into night," hospital director Dr. Pavel Nartov said. "It could hit at any time."

WHAT DETAILS HAVE EMERGED FROM NEGOTIATIONS BETWEEN RUSSIA AND UKRAINE?

Both sides are reporting some progress in negotiations.

Kremlin spokesman Dmitry Peskov said Thursday that "there should be contacts today" and that negotiators were breaking into working groups.

Zelenskyy's adviser Mikhailo Podolyak said Ukraine is demanding a cease-fire, the withdrawal of Russian troops and legal security guarantees for Ukraine from a number of countries. Ukrainian and Russian delegations held talks again Wednesday by video.

Another official in Zelenskyy's office, speaking on condition of anonymity because of the sensitivity of the talks, said 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 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.

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 Mariupol.

HOW IS THE WORLD RESPONDING TO THE WAR?

Six Western nations have called for a U.N. Security Council meeting on Ukraine on Thursday. The United Kingdom's U.N. Mission tweeted Wednesday that "Russia is committing war crimes and targeting civilians. Russia's illegal war on Ukraine is a threat to us all."

The mission said the meeting was requested by six Security Council members and posted their flags — Britain, the U.S., France, Ireland, Norway and Albania. It has not yet been officially scheduled.

Zelenskyy acknowledged in a speech Wednesday to the U.S. Congress that the no-fly zone may not happen. Still, he said the U.S. must sanction Russian lawmakers and block imports, in addition to providing military assistance.

"We need you right now," Zelenskyy said in remarks livestreamed Wednesday to the U.S. Capitol, which were punctuated with a graphic video showing the grisly aftermath of the invasion.

U.S. President Joe Biden announced that the U.S. is sending an additional $800 million in military assistance — including anti-aircraft and anti-armor weapons and drones — to Ukraine, making a total of $2 billion in such aid sent to Kyiv since Biden took office more than a year ago.

Biden and China's Xi Jinping are scheduled to speak by phone on Friday, and the U.S. president also plans to travel to Europe next week for talks with European leaders about the Russian invasion, and will attend an extraordinary NATO summit in Brussels. NATO has been bolstering its eastern flank with troops and equipment to deter Russia from invading any of its members.

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