The United Nations High Commissioner for Human Rights reported Sunday that it has verified 1,123 civilian casualties in Ukraine since the Russian invasion of the country began on Feb. 24.

The OHCHR said the casualties include 364 civilians that have been killed, including 25 children. Another 759 civilians have been injured in the fighting, including 41 children, the OHCHR reported.

OHCHR said it is only reporting verified civilian casualties and acknowledged that the real figures are likely "considerably higher."

Most of the civilian casualties recorded were caused by the use of explosive weapons with a wide impact area, including shelling from heavy artillery and multi-launch rocket systems, and missile and airstrikes.

At least 503 of the civilian casualties have occurred in the heavily Russian-controlled Donetsk and Luhansk provinces of eastern Ukraine's Donbas region, including 88 deaths. Another 374 civilian casualties, including 65 deaths, have occurred in the Ukrainian government-controlled areas of the Donbas region.

OHCHR reported that in other regions of Ukraine -- including the cities of Kyiv, Cherkasy, Chernihiv, Kharkiv, Kherson, Mykolaiv, Odesa, Sumy, Zaporizhzhia and Zhytomyr -- 276 civilians have died and 344 have been injured.

ABC News' Christine Theodorou

A Ukrainian lawmaker confirmed on Sunday that a gas pipeline was damaged in Mariupol, leaving more than 750,000 people in the second-largest city in eastern Ukraine's Donbas region without heat as temperatures plunged to 38 degrees.

Pavlo Kyrylenko, head of the Donetsk region administration, also confirmed Sunday's Mariupol civilian evacuation attempt failed after Russian troops shelled the city, breaking a temporary cease-fire agreement.

"It is extremely dangerous to take people out under such conditions," Kyrylenko wrote in a Facebook post.

Kyrylenko added that a column with a humanitarian cargo is headed to Mariupol from Zaporizhzhia.

Ukrainian lawmaker Inna Sovsun also tweeted a warning of a possible "humanitarian disaster" for nearly 1 million people in Mariupol. Sovsun expressed concern that people could freeze to death considering temperatures in the area often fall below zero this time of the year.

ABC News' Christine Theodorou

Ukraine President Volodymyr Zelenskyy has again appealed to Western countries to impose a "no-fly zone" over his country or send him military aircraft to use against Russia.

Zelenskyy on Sunday referred to an attack on the city of Vinnytsia in west-central Ukraine, saying he saw eight missiles hit an airport there.

"The airport is completely ruined. They deliberately destroy our infrastructure. They hit at our peaceful population. We plead with you every day and urge you again, all the world leaders, help us save lives. Impose a no-fly zone, close our skies. Help stop Russian missiles and aircraft dropping bombs," Zelenskyy said.

He added, "I know you can do it. It's your humanitarian duty to protect life and save lives. If you don't do it, help us with military aircraft! If you don't -- you will share blame for the loss of lives of innocent Ukrainians from now on.”

The Financial Times reported on Saturday night that the United States and Poland are working on a deal for Poland to provide Ukraine with fighter jets, which are Russian-made MiG jets.

-ABC News’ Patrick Reevell

Pope Francis on Sunday again made a strong appeal for peace in Ukraine during his noontime prayer at the Vatican.

“Rivers of blood and tears are flowing in Ukraine. This is not just a military operation, but a war, which sows death, destruction and misery,” the pontiff said from his window overlooking St. Peter’s Square. “The victims are ever more numerous, as are the people fleeing, especially mothers and children. In that tormented country the need for humanitarian assistance grows dramatically from hour to hour.”

People carrying Ukrainian and peace flags listened from the square.

“I make a heartfelt appeal to ensure that humanitarian corridors are in place, and that aid access to the besieged areas is guaranteed and facilitated, in order to offer vital assistance to our brothers and sisters oppressed by bombs and fear,” the pope said.

-ABC News’ Phoebe Natanson

The Russian forces charging toward Kyiv haven't made progress in the last day as they face Ukrainian resistance and low food and gas supply, a senior U.S. defense official told reporters Tuesday.

However, it could be a deliberate pause, the official said. "Part of the stall could be ... as a result of their own self-determined sort of pause in operations -- that they are possibly regrouping, rethinking, reevaluating," the official said.

The U.S. believes Russian forces "have committed now more than 80% of what was their pre-staged combat power," the official added.

The official said some Russian soldiers weren't told they were going into combat. The official said "not all of them were apparently fully trained and prepared."

The strong Ukrainian resistance has also hurt morale, according to the official.

Russia has now launched more than 400 missiles on Ukraine, the official said. The U.S. believes Russia has launchers that could be used for thermobaric weapons, but cannot confirm their use, the official said.

Russian forces are making the most progress in the south. Russians are attacking Kherson in south Ukraine, which "appears very much to be contested city at this point," the official said.

Russians are also approaching Mariupol in southeastern Ukraine, and while they haven't yet entered the city, "they are close enough now that they could attack Mariupol with long range fires," the official said.

Two towns on the path to Mariupol are believed to be occupied by the Russians, according to the official.

The U.S. believes the Russians hope to move north out of Mariupol up to the heavily-contested city of Kharkiv. The official said they believe Russian forces are trying to encircle Kharkiv.

The U.S. official noted that they've seen "certain risk-averse behavior by the Russian military" over the last week.

"Take the amphibious assault, for instance. They put those troops ashore a good 70 kilometers away from Mariupol because they knew Mariupol was going to be defended and they could put them ashore in an uncontested environment. And they still haven't reached Mariupol," the official said.

"They are not necessarily willing to take high risks with their own aircraft and their own pilots," the official said.

"And of course we're seeing that on the ground -- the fairly slow and steady progress that they have made, and you guys are seeing it for yourselves on the ground where ... units are surrendering, sometimes without a fight."

-ABC News' Matt Seyler