Defense Secretary Lloyd Austin told the Ukrainian minister of defense, Oleksii Reznikov, on Thursday that security assistance from the U.S. is now flowing into the region.

This includes aid from the $1 billion in security assistance recently announced by President Joe Biden, according to Pentagon press secretary John Kirby.

-ABC News' Matthew Seyler

British Prime Minister Boris Johnson on Thursday said consequences would be "very, very severe" if Vladimir Putin deploys chemical weapons.

"I think it would be catastrophic for him if he were to do that. And I think that he understands that," Johnson said.

He added that he thinks Biden was right in saying Putin's comments are a "clear sign" Russia is weighing the use of chemical weapons.

"I think [President] Joe Biden is right to warn ... because you know, when the Russians start doing this stuff about, 'oh, well … the factories in Ukraine producing American biological weaponry,' you know that is a prelude to a false flag operation and they could well do something, but I think it will make a profound and a disastrous mistake for Putin."

-ABC News' Christine Theodorou

President Joe Biden spoke to reporters Thursday, hours after a NATO emergency summit, saying that Russia's President Vladimir Putin "was banking on NATO being split," but NATO has never been more united than it is today.

"My early conversation with him in December and early January, it was clear to me he didn't think we could sustain this cohesion. NATO has never, never been more united than it is today. Putin is getting exactly the opposite of what he intended to have as a consequence of going into Ukraine," Biden said.

Biden has warned Russia may use chemical weapons in Ukraine and said that the U.S. and NATO would respond if they are used. Biden said the response would depend on the nature of the use of the weapons.

When asked whether this means they could cross the border into Ukraine, Biden said this is a decision they would make at the time.

The president said that he believes Russia should be removed from the G-20, but he raised the possibility that if removing Russia can’t be done because of "Indonesia and others" not in agreement, Ukraine should be able to attend future G-20 meetings.

Biden also spoke about the 100,000 Ukrainian refugees the U.S. will accept, with the caveat that they will be accepted "with a focus on reuniting families."

The United Nations General Assembly on Thursday voted to pass a resolution titled "Humanitarian consequences of the aggression against Ukraine," which explicitly names Russia as an aggressor party.

The resolution demands that Russia withdraw its military forces outside the internationally recognized borders of Ukraine.

It also condemns violations of humanitarian law and human rights abuses and seeks protections for civilians.

The resolution passed with 140 countries voting in favor, five voting against and 38 abstaining. The countries voting against the resolution were: Russia, Belarus, Syria, Eritrea and North Korea.

-ABC News' Zoha Qamar

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