Up to 1,200 people may have been inside the theater, said the city's Deputy Mayor Serhiy Orlov, as the Maxar satellite imagery firm said images from March 14 showed the word “children” had been written in large white letters in Russian in front of and behind the building.

The number of casualties is currently unknown, Ukrainian Foreign Minister Dmytro Kuleba tweeted, as Russian shelling in nearby neighborhoods continued.

"Another horrendous war crime in Mariupol. Massive Russian attack on the Drama Theatre where hundreds of innocent civilians were hiding. The building is now fully ruined. Russians could not have not known this was a civilian shelter," Kuleba tweeted.

In the northern city of Chernihiv, Ukraine's prosecutor-general said Russian forces killed 10 people who were waiting in line for bread, in the latest deadly attack on civilians in the three-week war.

Mariupol has been encircled by Russian forces, with an estimated 300,000 people trapped with no running water, electricity, or gas.

Some 400 staff and patients are still being held inside a Mariupol hospital, Ukrainian Deputy Prime Minister Iryna Vereshchuk said.

In Kyiv, Mayor Vitali Klitschko said Russian shelling damaged several residences in the city’s Podil neighborhood, just north of the city center and about 2.5 kilometers from the so-called “government quarter” that holds the presidential palace, president’s office and other significant offices.

The city remained under a 35-hour curfew early on March 16. Klitschko announced the restriction the day before, warning that, as Russian forces step up their strikes and close in on the capital, it faces a "difficult and dangerous moment."

U.S. President Joe Biden for the first time called Vladimir Putin a "war criminal" over the Russian president's bloody invasion of neighboring Ukraine.

"I think he is a war criminal," Biden told reporters on March 16, shortly after he announced an extra $800 billion weapons aid package for Ukraine.

The U.S. administration had avoided using the phrase until now, even when asked directly.

White House Press Secretary Jen Psaki said Biden was "speaking from his heart" after seeing images on television of "barbaric actions by a brutal dictator through his invasion of a foreign country."

"We have all seen barbaric acts, horrific acts by a foreign dictator in a country that is threatening and taking the lives of civilians, impacting hospitals, women who are pregnant, journalists, others," Psaki said.

Russia denies targeting civilians despite ample evidence to the contrary documented by the media.

In reaction to Biden's comment, Kremlin spokesman Dmitry Peskov said the statement was "unacceptable and unforgivable rhetoric," the TASS news agency said.

Putin ordered a large-scale invasion of Ukraine three weeks ago, alleging that Russia's actions are a "special military operation" to demilitarize and "denazify" Ukraine and topple its democratically elected government.

Biden announced U.S. help for Ukraine to acquire "additional longer-range anti-aircraft systems."

Biden said Ukraine will receive an additional 800 Stinger antiaircraft systems, 9,000 anti-tank weapons, as well as 100 drones, which Biden said "demonstrates our commitment to sending our most cutting-edge systems to Ukraine for its defense."

Ukrainian President Volodymyr Zelenskiy earlier asked the United States and its allies to declare a no-fly zone above Ukraine as Russian forces continued to pound civilians in several cities, including the capital Kyiv, as Moscow's unprovoked invasion reached the three-week mark.

“We need you right now,” Zelenskiy said in an emotional address on March 16 to the U.S. Congress via video link from Kyiv. "I call on you to do more.”

NATO chief Jens Stoltenberg also categorically ruled out any role for NATO in setting up and policing a no-fly zone over Ukraine to protect against Russian air strikes.

“NATO should not deploy forces on the ground or in the airspace over Ukraine because we have a responsibility to ensure that this conflict, this war, doesn’t escalate beyond Ukraine,” Stoltenberg said, speaking at the end of an emergency meeting of the alliance's defense ministers on March 16, .

Despite the sustained bombing of multiple civilian objectives by the Russian forces, video talks between Ukrainian and Russian negotiators appeared to be making some progress on March 16, with Russian Foreign Minister Sergei Lavrov calling the discussions "businesslike" and Zelenskiy describing Moscow's demands as becoming "more realistic."

Lavrov said on the Russian channel RBK TV that "serious" discussions on Ukraine having neutral status were taking place, with "concrete formulations that in my view are close to being agreed.”

Russia's top diplomat didn't elaborate on the discussions, but Ukraine's chief negotiator, Mykhaylo Podolyak, appeared to take a different view, saying that since a war with Russia is under way, "the model can only be Ukrainian and only about legally verified security guarantees."

"Ukraine has never been a militaristic state that attacks or plans to attack its neighbors, unlike some neighbors," he said in a statement on Telegram.

The UN’s highest court has ordered Russia to cease its military operations in Ukraine, with the justices noting that they were “profoundly concerned” by Moscow's unprovoked invasion of its neighbor last month.

In a ruling on March 16 in the Hague, the International Court of Justice (ICJ), the UN's top judicial tribunal, ordered Russia to cease its military operations in Ukraine, with the justices noting that they were “profoundly concerned” by Moscow's unprovoked invasion of its neighbor last month.

While its decisions are binding, Moscow is unlikely to heed the court’s ruling, which stemmed from a case filed by Kyiv over Russian allegations of genocide by Ukraine.

With reporting by AP, Reuters, and AFP