The Russian military is trying to subdue population centers "using more and more long-range fires, which are increasingly indiscriminate in terms of what they're hitting," a senior U.S. defense official warned Monday.

Russia has now launched more than 900 missiles against Ukraine, according to the official.

But the official said "almost all of Russia's advances remain stalled."

The Russians closest to Kyiv are still near Hostomel Airport, about 9 miles from the city center. Some troops are moving in behind those advance forces, "but not at a great pace," the official said.

The coastal city of Mariupol remains isolated and under heavy bombardment, with Russian forces to the north and east, though Ukrainians are continuing to fight back, the official said.

Significant fighting continues over Kharkiv in northeast Ukraine, with Russians relying more and more on long-range missile attacks, the official said.

The official said the U.S. is seeing a new line of advance, with 50 to 60 vehicles moving from the southwest of Kharkiv down toward the town of Izyum.

"The assessment is that they are trying to block off the Donbass area and to prevent the flow westward of any Ukrainian armed forces that would be in the eastern part of the country, prevent[ing] them from coming to the assistance of other Ukrainian defenders near Kyiv," the official said.

-ABC News' Matt Seyler

Pfizer said it won't stop delivering medicine to Russia, but will donate all profits from Russia to humanitarian support for Ukraine.

Pfizer also said it won't hold new trials in Russia and will stop recruiting new patients for its ongoing trials in the country.

Additionally, Pfizer said it "will cease all planned investments with local suppliers intended to build manufacturing capacity in the country."

-ABC News' Christine Theodorou

At least 636 civilians have been killed and another 1,125 injured in Ukraine since the attack began last month, the Office of the UN High Commissioner for Human Rights said.

These numbers are the verified deaths and injuries; actual death and injury figures are expected to be much higher, the OHCHR said.

Most of the casualties were due to explosive weapons impacting a wide area, including shelling, missiles and air strikes, the OHCHR said.

-ABC News' Christine Theodorou

Ukraine's presidential adviser Mykhailo Podolyak has tweeted that Ukraine and Russia are taking a "technical pause" in negotiations until Tuesday.

While the first three rounds of talks were held in Belarus, this fourth round is held remotely.

"Negotiations continue," Podolyak tweeted.

All Russian troops have left the Ukrainian cities of Kyiv and Chernihiv, withdrawing north toward the borders of Belarus and Russia to consolidate before likely redeploying to the Donbas region in eastern Ukraine, a senior U.S. defense official told reporters Wednesday.

But even with the Russians gone, the territory remains treacherous.

"There are some indications that they left behind mines and things like that, so the Ukrainians are being somewhat careful in some areas north of Kyiv as they begin to clear the ground and clear the territory and re-occupy it," the official said.

While the U.S. hasn't yet seen these troops redeploy elsewhere in Ukraine, it'll likely happen soon, according to the official. Ukrainian forces are preparing for a major fight in Donbas, the official said.

The official also said the Pentagon is "monitoring" an apparent nitric acid explosion in Ukraine's Luhansk region, which Russia blamed on Ukraine.

"We've seen the Russians claim that this was a Ukrainian attack on this. We do not believe that is true," the official said. "We do believe that the Russians are responsible, but exactly what they used when they did it, why they did it, what the damage is, we just don't have that level of detail," the official said.

The official also noted that a small number of Ukrainians currently in the U.S. for "professional military education" were pulled aside for a couple days of training on Switchblade drones, which the U.S. is sending overseas as part of its military aid, according to the official.

"Although it's not a very difficult system to operate, we took advantage of having them in the country to give them some rudimentary training on that," the official said.

-ABC News' Matt Seyler