President Recep Tayyip Erdoğan will attend NATO's upcoming extraordinary leaders' summit in Brussels, a statement from the Presidency said.

Erdoğan is due in Belgium on March 23, the statement added.

"The summit will focus on precautions to strengthen NATO's deterrence and defense structure," the Presidency added.

"Views regarding the steps to shape the future of the alliance will be exchanged during the summit," the statement also said, also mentioning the later summit set for June in the Spanish capital Madrid. Erdoğan will hold talks with several heads of state and government on the sidelines of the summit.

The summit comes amid Russia's ongoing invasion of Ukraine.

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.