Russian forces west of Kyiv are “digging in” and moving into a "defensive position," according to a senior U.S. defense official.

"It's not that they're not advancing, they're actually not trying to advance right now," the official told reporters Wednesday. "They're taking more defensive positions."

East of Kyiv, Ukrainian forces near Brovary have pushed the Russians "back to about 55 km" (roughly 34 miles) east and northeast of Kyiv, according to the official. That update comes a day after the official said Russian forces had stalled at between 20 and 30 miles east and northeast of Kyiv.

Ukrainian authorities claimed earlier Wednesday that they have managed to encircle the Russian forces that had advanced on Kyiv and were in some key towns on the edge of the capital. They said they had managed to push back Russian troops from the northwestern town of Irpin, although it was still being shelled.

-ABC News' Luis Martinez and Patrick Reevell

Russia's Defense Ministry claimed on Wednesday that its forces have advanced another 2 kilometers and are fighting against units of Ukraine's 54th Separate Mechanized Brigade in the southern and southeastern outskirts of Novo-Mikhaylovka.

Russia said troops it backs in Donetsk inflicted fire damage to Ukraine's units of the 25th Airborne Brigade fighting for the capture of Kamenka and Novobakhmutovka.

Russia claimed it hit 86 Ukrainian military assets, among them six command posts; two rocket-launch systems and 49 areas of concentration of equipment and military hardware.

The ministry also claimed Russian forces shot down nine Ukrainian unmanned aerial vehicles near Izyum, Kiev, Sumy, Kharkov and Chernigov.

Russia said it destroyed 255 unmanned aerial vehicles, 189 anti-aircraft missile systems, 1,564 tanks and other armored combat vehicles, 158 multiple launch rocket systems, 612 field artillery and mortars, as well as 1,367 units of special military vehicles since the beginning of the invasion.

The International Atomic Energy Agency is ready to send experts and equipment to Ukraine to help ensure the safety and security of its nuclear facilities and prevent the risk of a severe accident, Director General Rafael Mariano Grossi said Wednesday.

In a video statement, Grossi said he is gravely concerned about the situation and stressed the urgent need to conclude an agreed framework that would enable the IAEA to provide technical assistance for the safe and secure operation of Ukraine’s nuclear facilities, which include 15 reactors a well as the Chernobyl site.

"I have personally expressed my readiness to immediately come to Ukraine to conclude such an agreement, which would include substantial assistance and support measures, including on-site presence of IAEA experts at different facilities in Ukraine, as well as the delivery of vital safety equipment," Grossi said.

He added: "I hope to be able to conclude this agreed framework without further delay. We cannot afford to lose any more time. We need to act now."

-ABC News' Rashid Haddou

White House National Security Adviser Jake Sullivan said Wednesday that the U.S. is looking at the “deliberate drumbeat of misinformation” and propaganda from Moscow, which has “all the markers of a precursor” for them to use chemical weapons.

Earlier, President Joe Biden said there is a "real threat" of Russia using chemical weapons in Ukraine.

Sullivan declined to comment on whether Biden's assessment was based on movements of weapons, or if it was based entirely on state propaganda.

Sullivan also commented on negotiations between Russia and Ukraine and said there are questions about Russia's trustworthiness.

"I will point out that Russia has not been trustworthy in its public statements about its intentions with respect to Ukraine for months. So, we take everything that they say at the negotiating table, or from their podiums, with a very large grain of salt," he said, talking to reporters on Air Force One.

-ABC News' Justin Gomez

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