Russia-Ukraine updates: 2 US veterans who joined Ukrainian forces missing

Multiple rockets struck the center of Ukraine's second-largest city on Sunday, killing at least five people and injuring 13, Ukrainian officials said.

Kharkiv, in northeast Ukraine, has been targeted by Russian shelling since the early days of the invasion, and Sunday was no exception. The barrage of rockets hit apartment buildings and left broken glass, debris and the part of at least one rocket scattered on the streets of Kharkiv, according to Ukrainian regional officials.

Firefighters and residents scrambled to douse flames in several apartments that caught fire.

A kindergarten school in the city was also damaged in the attack.

"There were four shells, one of them fell between the fence and the kindergarten. It turned everything upside down," a staff member at the school told reporters.

It was the second consecutive day of heavy shelling in the center of Kharkiv. The city's mayor, Ihor Terekhov, said three people were killed and 34 others were wounded by Russian missile strikes on Saturday.

One missile hit a community kitchen set up in Kharkiv by World Central Kitchen, a nonprofit organization run by celebrity chef Jose Andres. Four World Central Kitchen staff members were injured in the attack. Andres posted a message on Twitter reporting the staff members suffered non-life-threatening injuries and are going to be fine.

Pope Francis made an anguished Easter Sunday plea for peace in the "senseless" war in Ukraine.

During Mass at the Vatican, the pontiff used the occasion to condemn Russia's invasion of Ukraine.

"May there be peace for war-torn Ukraine, so sorely tried by the violence and destruction of this cruel and senseless war into which it was dragged," Francis said.

The Vatican estimated there that about 50,000 people made the pilgrimage to St. Peter's Square on Sunday to celebrate Easter Mass, while police said the number in attendance was closer to 100,000.

Although he mostly walked unaided, the pope was seen having difficulty walking on live broadcast of the event.

"In this terrible night of suffering and death, may a new dawn of hope soon appear. Let there be a decision for peace," the pope said. "May there be an end to the flexing of muscles while people are suffering. Please, let us not get used to war. Let us all commit ourselves to imploring peace, from our balconies and in our streets. May the leaders of nations hear people’s plea for peace."

Francis added, "I hold in my heart all the many Ukrainian victims, the millions of refugees and internally displaced persons, the divided families, the elderly left to themselves, the lives broken and the cities razed to the ground."

The Russian Defense Ministry issued an emergency statement calling on the remaining Ukrainian forces at the Azovstal plant in Mariupol to lay down their arms and halt fighting on Sunday morning.

“Taking into account the catastrophic situation at Azovstal Iron and Steel Works and being guided by purely humane principles, the Russian Armed Forces suggest that the militants of the nationalist battalions and foreign mercenaries stop any hostilities and lay down their arms from 06:00 (Moscow time) on April 17. All those who lay down their arms are guaranteed life,” the ministry said in a statement.

Earlier, Ukrainian President Zelenskyy said in an interview with Ukrainian media Ukrainska Pravda that the continuing siege of the southeastern city could scuttle attempts to negotiate an end to the war.

"The destruction of all our guys in Mariupol -- what they are doing now -- can put an end to any format of negotiations," he said.

-ABC News' Tanya Stukalova and Jason Volack

Four staff members of World Central Kitchen were injured in a shelling in Kharkiv in northeast Ukraine on Saturday, the organization said.

"Food heroes them all!" chef and founder Jose Andres said on Twitter. "Relief that so far that just were 4 wounded…..praying for them!"

The "Russian attacks must stop against civilian buildings and markets and churches and schools," he added.

The organization, which responds to disaster situations, has been providing meals to refugee families leaving Ukraine.

World Central Kitchen CEO Nate Mook said a missile struck one of the World Central Kitchen's partner restaurants in Kharkiv, which has seen heavy shelling.

"As you can see, tremendous amounts of damage," Mook said in a video posted to Twitter Saturday from the scene, standing before debris and burned-out cars.

Mook said "a number" of staff were wounded and hospitalized, and that one person not affiliated with the organization was killed.

"Just a tremendous amount of carnage left behind for no reason," he said. "Just absolutely horrific brutality."