At least 1,189 civilians have been killed and 1,901 others have been injured in Ukraine since Russian forces invaded on Feb. 24, according to the latest figures from the Office of the United Nations High Commissioner for Human Rights (OHCHR).

At least 108 children were among the dead and 142 among the injured, according to the OHCHR, which noted that the Prosecutor General's Office of Ukraine had reported at least 145 children were killed and 222 injured as of Wednesday.

"We know the actual figures are likely far higher," U.N. High Commissioner for Human Rights Michelle Bachelet said in a statement Wednesday. "In many places of intensive hostilities, such as Mariupol and Volnovakha, it is very challenging to obtain a comprehensive picture."

According to a press release from the OHCHR, 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 multiple launch rocket systems, as well as missile and airstrikes.

The agency has also received "credible allegations that Russian armed forces have used cluster munitions in populated areas at least 24 times," according to Bachelet, who noted that her office is "also investigating allegations that Ukrainian armed forces have used such weapons."

"The persistent use of explosive weapons with wide area effects in populated areas is of immense concern," she said. "Homes and administrative buildings, hospitals and schools, water stations and electricity systems have not been spared."

According to Bachelet, at least one Ukrainian facility for bedridden patients and others with disabilities, mostly elderly people, came under fire while its residents were inside, with dozens of alleged casualties.

"My colleagues in Ukraine are working to establish the fate and whereabouts of survivors," she added.

The OHCHR noted in its press release that "the receipt of information from some locations where intense hostilities have been going on has been delayed and many reports are still pending corroboration." Those areas include Mariupol and Volnovakha in the Donetsk Oblast, Izium in the Kharkiv Oblast, Popasna in the Luhansk Oblast, and Trostianets in the Sumy Oblast, where there are allegations of numerous civilian casualties. Casualty numbers from those locations are being further corroborated and thus are not included in the latest statistics, according to the agency.

Russian President Vladimir Putin has apparently "massively misjudged" his invasion of Ukraine, a U.K. intelligence chief said Thursday.

"It's clear he misjudged the resistance of the Ukrainian people. He underestimated the strength of the coalition his actions would galvanize. He underplayed the economic consequences of the sanctions regime, and he overestimated the abilities of his military to secure a rapid victory," Jeremy Fleming, head of the U.K. Government Communications Headquarters (GCHQ), said during a speech in Australia's capital, Canberra.

"We've seen Russian soldiers, short of weapons and morale, refusing to carry out orders, sabotaging their own equipment and even accidentally shooting down their own aircraft," he added.

While Fleming agreed with a recent assessment by U.S. intelligence that Putin's advisers were believed to be too afraid to tell the truth, he said the "extent of these misjudgments must be crystal clear to the regime." He warned that Russia is searching for cyber targets and bringing in mercenaries to reinforce its stalled military campaign in Ukraine.

In his daily address Wednesday, Ukrainian President Volodymyr Zelenskyy said he held a very "detailed" conversation with President Joe Biden on various topics.

Zelenskyy said he thanked Biden for $1 billion in new humanitarian aid and an additional $500 million in direct support.

Zelenskyy said he stressed the current moment is a turning point

"I told President Biden what Ukraine needs, and I was as sincere as possible with him," he said. "The support of the United States is vital for us, and now it is especially important to lend a hand to Ukraine, to show all the power of the democratic world."

The Ukrainian president said he reiterated his calls for more weapons and resources to fight Putin's forces.

He also said he didn't buy Russian's "withdrawal" from Kyiv and Chernihiv.

"We do not believe anyone," he said. "We will not give up anything, and we will fight for every meter of our land, for every our person."

-ABC News' Christine Theodorou

Over the last 24 hours, the Pentagon has seen less than 20% of the Russian troops that had been around Kyiv "reposition" northward, Pentagon spokesman John Kirby said.

Kirby said Russian claims that they are withdrawing troops to deescalate fighting amid peace talks with Ukraine is just "spin."

Kirby said he was intentionally using the term "reposition" instead of withdrawal to describe the movement of Russian forces "because the way it's being spun by the Ministry of Defense is that they're pulling back and they're trying to deescalate and depressurize the situation. And we just don't believe we haven't seen any evidence of that."

Kirby said the U.S. assesses that the Russians are instead “going to refit these troops, resupply them, and then probably ... employ them elsewhere in Ukraine."

-ABC News' Luis Martinez

About 631 residents of the bombed-out city of Mariupol in southeast Ukraine were able to evacuate on Thursday, according to a Ukrainian official.

Ukrainian Deputy Prime Minister Iryna Vereshchuk said the residents fled the city in private vehicles after 45 buses reserved to drive them out failed to make it into the city. Vereshchuk said another 600 civilians still in Mariupol plan to try to evacuate again on Friday.

Ukrainian President Volodymyr Zelenskyy said Thursday in an address to lawmakers of Australia, the Netherlands and Belgium that more than 90% of all buildings in Mariupol have been completely destroyed by Russian strikes.

"Thousands of peaceful Mariupol residents died, people are buried just in the city, in the courtyards of high-rise buildings, or rather, what is left of the high-rise buildings," Zelenskyy said.

-ABC News' Christine Theodorou