A second round of talks between Russian and Ukrainian negotiators will be held at the previously planned venue in neighboring Belarus on Thursday at around 3 p.m. local time (7 a.m. ET), according to Vladimir Medinsky, head of the Russian delegation and aide to Russian President Vladimir Putin.

"The talks will take place -- we are now in contact with the Ukrainian side -- at the same venue where they were planned, on the territory of the Brest region of Belarus," Medinsky told reporters Thursday, adding that Russian negotiators are "waiting calmly."

"I think the talks will begin at 3 p.m.," he said.

Ukraine's presidential advisor Mykhailo Podolyak from the Ukrainian delegation later posted a photo of himself on his official Twitter account on Thursday, saying they were en route via helicopter to the talks in Belarus

-ABC News' Patrick Reevell and Tanya Stukalova

Russian Foreign Minister Sergey Lavrov wouldn't comment on civilian deaths from Russia's invasion of Ukraine when pressed during an interview Thursday with ABC News' George Stephanopoulos on "Good Morning America."

"I cannot comment," Lavrov said, adding that there are "a great deal" of "conjectures."

Ukraine claimed Thursday to have raised its flag over the town of Bucha, close to the Ukrainian capital where some of the most intense fighting has been taking place in recent days and where Russia's push south on Kyiv appears to have stalled.

A video posted on the official Facebook page of the Ukrainian Armed Forces' ground troops purportedly shows soldiers hoisting the national flag outside Bucha's town hall. The town is just a few miles north of the edge of Kyiv and about 15 miles from the center of the capital. Fighting is reported to be ongoing nearby and, in the video, an explosion can be heard in the distance as they raise the blue and yellow flag.

-ABC News' Yulia Drozd

Ukraine is asking the International Atomic Energy Agency (IAEA) to call on NATO to close access to the airspace over the country's Chernobyl nuclear power plant and the surrounding exclusion zone.

The deserted exclusion zone around the Chernobyl nuclear power plant, where the world's worst nuclear accident took place in 1986, was seized by Russian forces last week.

A joint appeal to the IAEA was signed Wednesday by Ukrainian Energy Minister Herman Galushchenko, Oleh Korikov, head of the State Nuclear Regulatory Inspectorate of Ukraine, and Petro Kotin, head of Ukraine's state nuclear energy company Energoatom.

"The fact of the seizure of the world-famous Chernobyl nuclear power plant has all the hallmarks of an act of nuclear terrorism committed against Chernobyl nuclear facilities and its personnel by Russian military units," they said in the appeal.

-ABC News' Yulia Drozd

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