The first deliveries from the $800 million military assistance that U.S. President Joe Biden authorized for Ukraine a week ago have started to arrive, a White House official told ABC News on Wednesday.

The military aid package includes Stinger anti-aircraft systems, Javelin anti-armor weapons, light anti-armor weapons, AT-4 anti-armor systems and tactical unmanned aerial systems.

CNN was first to report the deliveries.

-ABC News' Justin Gomez

The governor of Ukraine's Chernihiv Oblast, Viacheslav Chaus, claimed Wednesday that Russian forces have destroyed the bridge linking the region to the Ukrainian capital, Kyiv.

In a video posted on Telegram, Chaus shows the destroyed bridge over the Desna river, which he said effectively means that the road from Chernihiv to Kyiv is now severed.

-ABC News' Fergal Gallagher

Russia and Ukraine have agreed on nine humanitarian corridors to try to evacuate civilians trapped in embattled Ukrainian towns and cities on Wednesday, according to Ukrainian Deputy Prime Minister Iryna Vereshchuk.

But the agreement does not include a safe passage from the heart of Mariupol, Vereshchuk said in an address Wednesday, adding that she hopes people wishing to leave the besieged southeastern port city can make it to nearby Berdyansk, where humanitarian aid awaits them. She said 24 buses are on standby to transport people.

Some of the previous attempts to evacuate civilians from Mariupol have failed after Russian forces continued to shell the city, despite agreeing to temporary cease-fires.

Russia warned Wednesday that sending NATO peacekeepers to Ukraine would be "a very reckless and extremely dangerous decision."

Last week, Polish Prime Minister Mateusz Morawiecki announced that Poland will formally submit a proposal at the NATO summit on Thursday for a peacekeeping mission in Ukraine amid Russia's invasion.

Kremlin spokesman Dmitry Peskov commented on Poland's plans while speaking to reporters Wednesday.

"It would be a very reckless and extremely dangerous decision," Peskov said. "A special military operation is going on, and any possible contact by our troops with NATO troops can lead to quite clear consequences that would be hard to repair."

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