The United Kingdom is imposing sanctions on Belarus in response to the role the country is playing in Russia’s invasion of Ukraine, including facilitating the invasion from within its borders.

Four senior defense officials have been sanctioned, leaving them unable to travel to the U.K. and freeing any of their U.K.-based assets. Also sanctioned are a Belarusian aircraft repair plant and a military semiconductor manufacturer.

Britain's Foreign Secretary Liz Truss said, "We are inflicting economic pain on [Russian President Vladimir] Putin and those closest to him. … The [Belarusian President Alexander] Lukashenko regime actively aids and abets Russia’s illegal invasion and will be made to feel the economic consequences for its support for Putin."

ABC News' Christine Theodorou

The tower that provides the main signal to TV and radio in Kyiv has been struck by a Russian missile, leaving at least five dead, Ukrainian officials said.

Ukraine's Foreign Minister Dmytro Kuleba tweeted that the tower "is situated on the territory of Babyn Yar. On September 29-30, 1941, Nazis killed over 33 thousand Jews here. 80 years later, Russian Nazis strike this same land to exterminate Ukrainians. Evil and barbaric."

Ukraine's President President Volodymr Zelenskyy tweeted, "What is the point of saying «never again» for 80 years, if the world stays silent when a bomb drops on the same site of Babyn Yar? At least 5 killed. History repeating."

President Joe Biden and Zelenskyy discussed the attack on the TV tower that impacted the Babyn Yar Holocaust memorial on their call Tuesday, according to the White House.

-ABC News' Christine Theodorou

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

A spokesperson for the U.N. High Commissioner for Human Rights said 136 civilians have been killed in Ukraine, including 13 children.

Another 400 Ukrainian civilians have been wounded, spokesperson Liz Throssell said.

NATO Secretary-General Jens Stoltenberg warned Wednesday that Russia has still shown no signs of de-escalation on the ground, despite claims of withdrawing troops from near Ukraine.

"We have heard the signs from Moscow about the readiness to continue diplomatic efforts, but so far we have not seen any de-escalation on the ground," Stoltenberg said during a press conference outside NATO headquarters in Brussels. "On the contrary, it appears that Russia continues their military build-up."

NATO continues to monitor the situation "very closely," as it remains unclear whether Russia is pulling back, according to Stoltenberg.

"What we see is that they have increased the number of troops and more troops are on their way," he told reporters. "So, so far, no de-escalation but of course, we hear also the message about diplomacy and we are ready to engage in diplomatic efforts with Russia."

Stoltenberg explained that the Russian military has "always moved forces back and forth."

"So, just that we see movement of forces of battle tanks doesn't confirm a real withdrawal," he said. "It has been a big up and down, back and forth, all the way, but the trend over the last weeks and months has been a steady increase in the Russian capabilities close to Ukraine's borders. So Russia retains the capability of a full-fledged invasion of Ukraine without any warning time."

As of Wednesday morning, there were still approximately 100,000 Russian troops at Ukraine's borders, including in Belarus, according to Stoltenberg, who said he hopes the dialogue with Russia will continue and that the claims of withdrawal are true "because nobody has any interest in confrontation or war in Europe."

"We are at a pivotal moment in the history of Ukraine, Europe and global stability," he added. "We look forward to seeing evidence of the withdrawal of troops on Russia's part, but we need to prepare for any eventuality, with that significant escalation of Russian troops that we have seen over the last weeks and months."

Stoltenberg's comments came ahead of a meeting of the military alliance's defense ministers, including U.S. Secretary of Defense Lloyd Austin, to discuss the current crisis.