Russian commanders in Mariupol "will be concerned by the time it is taking to subdue" the Ukrainian port city, the U.K. Ministry of Defense said Monday in an intelligence update.

"Concerted Ukrainian resistance has severely tested Russian forces and diverted men and materiel, slowing Russia's advance elsewhere," the ministry said.

Mariupol, a strategic port in eastern Ukraine's Donetsk Oblast, has been under heavy Russian bombardment since the start of the invasion on Feb. 24. Strong resistance from Ukrainian troops has prevented Russian forces from taking full control of the city.

Despite Russia's claims that it would not strike Ukrainian cities or threaten civilian lives, "the targeting of populated areas within Mariupol aligns with Russia's approach to Chechnya in 1999 and Syria in 2016," according to the ministry.

"The effort to capture Mariupol has come at significant cost to its residents," the ministry added. "Large areas of infrastructure have been destroyed whilst the population has suffered significant casualties."

The mayor of Kharkiv, Igor Terekhov, lashed out at Russia for continuing its onslaught on the city despite it being Orthodox Palm Sunday.

"Dear Kharkiv citizens, today is the 53rd day of war, the war in which we are defending our lives and honor fighting against the ruthless army of the aggressor," Terekhov said in a recorded address translated from Russian by The Associated Press.

"And also today is a big Orthodox holiday, Palm Sunday. But it looks like those who wear the letter Z do not have a cross on them," he added, referring to the omnipresent letter painted on Russian tanks.

Kharkiv, Ukraine's second-largest city and a key outpost in the country's east, has been under intense shelling for days. Several people were killed in attacks on Sunday.

"Even days that are sacred to Christians are no reason for the enemy to lessen bombardments on Kharkiv," Terekhov said.

The vast majority of people in both Russia and Ukraine are Orthodox Christians. Holy Week began on Sunday with Easter coming on April 24. Many in Ukraine, including Kharkiv, celebrated Palm Sunday in bomb-damaged churches or buildings without electricity.

"I congratulate you all, my fellow Kharkiv citizens, with Palm Sunday, and on the eve of the Holy Week, I want to wish fortitude to all of us," Terekhov said. "The enemy is testing our resolve, let us show them that Kharkiv citizens will not be broken or intimidated."

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."

At least one person -- a journalist -- was killed in a rocket attack on a residential building in Kyiv on Thursday evening, ABC News has learned.

Kyiv Mayor Vitali Kilitschko said Friday that rescuers had found the body of a victim amid the rubble.

Radio Liberty, a service of the U.S.-funded Radio Free Europe/Radio Liberty, reported that one of its employees, Vira Gyrych, was killed when a Russian missile hit her apartment in the Ukrainian capital on Thursday. Her body was found beneath the wreckage Friday morning, according to the report.

Gyrych had worked as a journalist and producer for Radio Liberty's Kyiv bureau since 2018. Prior to that, she worked for leading Ukrainian television channels, according to Radio Liberty.

"The editorial staff of Radio Liberty expresses its condolences to the family of Vira Gyrych and will remember her as a bright and kind person, a true professional," Radio Liberty said in its report.

Israeli Ambassador to Ukraine Michael Brodsky also confirmed Gyrych's death in a Twitter post, saying she was a former employee of the Israeli embassy in Kyiv.

Thursday's rocket attack came as United Nations Secretary-General Antonio Guterres visited Kyiv. Five Russian missiles flew into the city, according to Ukrainian President Volodymyr Zelenskyy. At least 10 people were injured, including four who were hospitalized, according to the Kyiv City Council.