On Thursday, international outrage over Russia's invasion of Ukraine intensified when US and Ukrainian officials alleged Russian forces killed civilians waiting in line for food and refuge in a theater, multiple reports said Thursday.

U.S. President Joe Biden labeled Russian President Vladimir Putin a "war criminal" in remarks the Kremlin described as "unforgivable," as the Kremlin said the war in Ukraine was "going according to plan" in the face of rumors of peace talks.

Despite the largest assault on a European state since World War II, Moscow has yet to conquer any of Ukraine's major cities. As the battle enters its fourth week, more than 3 million Ukrainians have fled and hundreds have died.

According to Ukraine's foreign ministry, Russian soldiers dropped a heavy bomb on a theatre in Mariupol, a beleaguered southern port city, trapping numerous civilians and killing an unknown number of people, reports disclosed. 

The word "children" was scrawled in bold Russian lettering on the ground outside the red-roofed building, a satellite imagery shows. 

Human Rights Watch said the theater housed at least 500 civilians.

"This raises major concerns about the intended target in a city where inhabitants have already been under siege for days and telecommunications, power, water, and heating have been cut off nearly entirely," said Belkis Wille of the rights group.

The Russian news agency RIA said Moscow denies targeting civilians and the Russian defense ministry says its soldiers did not strike the structure.

Thirteen buses carrying over 300 Mariupol refugees landed in Russia's Rostov area on Thursday, Russia's Interfax news agency reported.

The U.S. Embassy in Kyiv said Russian forces shot and killed ten individuals in Chernihiv, northeast of Kyiv, who were standing in line for food. Russia has denied the strike, claiming it was a fabrication.

On Wednesday, the United Nations' top court for international disputes ordered Russia to halt its military activities in Ukraine, saying it was "extremely worried" by Moscow's use of force. 

Ukraine's military forces are undertaking small-scale counter-attacks on multiple fronts, while Russian troops are unable to gain progress due to a lack of resources.

At the current talks between Russia and Ukraine, there were signs of compromise and development.

Negotiators, according to the Kremlin, are contemplating a status for Ukraine akin to that of Austria or Sweden, both European Union members who are not members of NATO's military alliance.