The leaders of the United States, Germany, France, Britain and Italy spoke to each other by telephone on Monday and agreed on the importance of remaining united over Ukraine amid the country's invasion by Russia, Italian Prime Minister Mario Draghi's office said.

"The leaders reaffirmed the importance of the unity of purpose and action shown in the face of the war in Ukraine and its repercussions," the statement said.

"In the face of the grave humanitarian emergency, the leaders pledged to coordinate efforts to help the Ukrainian population fleeing the conflict or those stranded at home," it added.

Italy said earlier on Monday that the call was aimed at preparing for NATO, Group of Seven and European Council meetings planned for later this week.

Three weeks into the invasion, the two sides now seem to be trying to wear down the other, experts say, with bogged-down Russian forces launching long-range missiles at cities and military bases as Ukrainian forces carry out hit-and-run attacks and seek to sever Russian supply lines.

"The block-by-block fighting in Mariupol itself is costing the Russian military time, initiative and combat power," the Washington-based Institute for the Study of War said in a briefing.

U.S. Defense Secretary Lloyd Austin said Ukrainian resistance means Russian President Vladimir Putin’s "forces on the ground are essentially stalled."

"It’s had the effect of him moving his forces into a woodchipper," Austin told CBS on Sunday.

The strike on the art school was the second time in less than a week that officials reported an attack on a public building where Mariupol residents had taken shelter. On Wednesday, a bomb hit a theater where more than 1,000 people were believed to be sheltering.