Ukrainian authorities said on Friday that 300 people died in the Russian airstrike of a theater in Mariupol last week.

More than 1,300 residents of the besieged city had taken shelter there, according to the Ukrainian Parliament’s human rights commissioner. The theater’s basement contained a bomb shelter.

The Russian word for “children” was written in large letters outside two of the theater’s entrances in order to warn Russian forces that it was a civilian refuge that shouldn’t be attacked. The Russians bombed it anyway.

Mariupol’s government cited eyewitnesses in announcing the death toll.

“We still do not want to believe in this horror. We still want to believe that everyone managed to escape. But the words of those who were inside the building at the time of this terrorist act say otherwise,” the government wrote on its official Telegram channel.

If confirmed, it would be the deadliest attack on a civilian target in the course of Russia’s invasion of Ukraine, now entering its fifth bloody week. It is unclear if rescue teams are still looking for survivors at the site.

The Associated Press provides key context:

…the emerging picture is certain to fuel allegations Moscow has committed war crimes by killing civilians, whether deliberately or by indiscriminate fire. And it could increase pressure on NATO to step up military aid. The alliance has refused so far to supply warplanes or establish a no-fly zone over Ukraine for fear of getting into a war with Russia.

U.S. President Joe Biden’s national security adviser, Jake Sullivan, said Friday the reaction to the theater bombing was “just absolute shock, particularly given the fact that it was so clearly a civilian target.” He said it showed “a brazen disregard for the lives of innocent people.”

CBS News reports on the conditions in Mariupol:

The United Nations said Friday that it was receiving mounting evidence of mass graves in Mariupol, including one that appeared to be holding about 200 victims of the war. Ukrainian officials estimate there are still roughly 100,000 civilians trapped in the city, where Russia’s siege cut off food, water, gas and electricity supplies weeks ago. 

Some 10 million Ukrainians have been forced to flee their homes after just one month of war, with at least 3.5 million driven to seek safety in neighboring countries. More than half of them have crossed Ukraine’s western border into Poland, where President Joe Biden was paying a visit on Friday in a show of solidarity both with the refugees, and those helping them.

Earlier this week, the U.S. government said Russia has committed war crimes.

“Today, I can announce that, based on information currently available, the U.S. government assesses that members of Russia’s forces have committed war crimes in Ukraine,” said Secretary of State Antony Blinken in a statement.

“We are committed to pursuing accountability using every tool available, including criminal prosecutions,” he added.