The mood at NATO's emergency summit in Brussels has been "sober" and "resolute" so far, according to senior U.S. administration officials.

"There was a very strong sense that that we are facing a significant historical moment, and very strong support from all the leaders who spoke about the need to defend our democracy," one of the officials told reporters during a telephone briefing Thursday.

The officials said Ukrainian President Volodymyr Zelesnkyy, who addressed NATO leaders remotely, spoke "very eloquently" and "repeated his requests for continued and increased Western security assistance."

"But notably, there was not a request for a no-fly zone," an official added. "There was also not a request for NATO membership."

U.S. President Joe Biden was the first NATO head of state to speak after Zelenskyy's speech and noted that Thursday marks one month since Russia launched its invasion of Ukraine. He discussed the sanctions that have already been imposed and expressed strong support for Ukraine as well as humanitarian and military aid and reaffirmed his support for Article 5, which is the cornerstone of NATO and states that an attack on one member is an attack on all members.

China was also a topic of discussion among "many of the speakers," officials said, as the world watches whether Beijing offers any military or economic assistance to Moscow.

"We need to continue to call on China not to support Russia in its aggression against Ukraine, and that we need China to call for a peaceful end of the conflict as a responsible member of the international community," an official told reporters.

NATO allies are now consulting about "providing anti-ship missiles to Ukraine," following the Ukrainian navy's attack on a Russian ship earlier Thursday, though officials noted "there may be some technical challenges with making that happen."

When asked whether there were discussions about NATO responding to a potential chemical attack by Russian forces in Ukraine, the officials told reporters: "Yes, there were some references to that."

"It's something that NATO as a military alliance is already postured to do," one official said, "and it's something that they're recognized that they need to continue to do given the various scenarios that could emerge as part of this conflict."

-ABC News' Justin Gomez

Ukrainian President Volodymyr Zelenskyy accused Russia of using phosphorous bombs on civilians in Ukraine on Thursday.

"This morning, by the way, there were phosphorus bombs -- phosphorus Russian bombs. Adults were killed again and children were killed again," Zelenskyy told NATO leaders via video link from Kyiv, during an emergency NATO summit in Brussels. "I just want you to know that the alliance can still prevent the deaths of Ukrainians from Russian strikes, from Russian occupation, by providing us with all the weapons we need."

Phosphorus munitions inflict excruciating burns and can lead to infection, shock and organ failure.

-ABC News' Julia Drozd and Joe Simonetti

Ukrainian President Volodymyr Zelenskyy warned NATO leaders on Thursday that Russia's offensive will ultimately go beyond Ukraine, unless the Western defense alliance takes stronger action.

"Ukraine never wanted this war and does not want to fight for years. We just want to save our people," Zelenskyy said in an impassioned speech via video link from the Ukrainian capital, Kyiv. "I am sure you already understand that Russia does not intend to stop in Ukraine -- does not intend and will not. It wants to go further."

As all 30 NATO heads of state meet in Brussels for an emergency summit to discuss their response to Russia's invasion of Ukraine, Zelenskyy lamented that "the worst thing during the war is not having clear answers to requests for help."

"On Feb. 24, I addressed you with a perfectly clear, logical request to help close our skies -- in any format. Protect our people from Russian bombs and missiles. We did not hear a clear answer," he told NATO leaders. "And you see the consequences today -- how many people were killed, how many peaceful cities were destroyed."

"I have been repeating the same thing for a month now: To save people and our cities, Ukraine needs military assistance without restrictions, as Russia uses without restrictions its entire arsenal against us," he continued. "Ukraine asked for your planes so that we do not lose so many people. And you have thousands of fighter jets, but we haven't been given any yet."

"We asked for tanks so that we can unblock our cities that are now dying," he added. "You have at least 20,000 tanks. Ukraine asked for a percent -- 1% -- of all your tanks to be given or sold to us. But we do not have a clear answer yet."

Zelenskyy criticized NATO for "worrying about how Russia will react" but said he wants "to be clear" that he does not "blame" the alliance.

"It's not your missiles, it's not your bombs that are destroying our cities," he told NATO leaders. "I just want you to know that the alliance can still prevent the deaths of Ukrainians from Russian strikes, from Russian occupation, by providing us with all the weapons we need."

While Ukraine has been refused NATO membership, "the most powerful defense alliance in the world," Zelenskyy noted how his country has been defending "all our common values" for the last month.

"Yes, we are not in the alliance," he said. "But Ukrainians never thought that the alliance and the allies were different."

After a month of war, Zelenskyy said the "only thing" he demands from NATO now is: "Never, please, never tell us again that our army does not meet NATO standards."

-ABC News' Julia Drozd and Joe Simonetti

Russia claimed Thursday that its troops have seized Izyum, a city in eastern Ukraine's Kharkiv Oblast.

The Russian Ministry of Defense said in a statement that units of the Russian Armed Forces had taken control of the city by Thursday morning.

Izyum is about 75 miles southeast of the oblast capital, Ukraine's second-largest city of Kharkiv, which has faced relentless bombardment by Russian forces since the start of the invasion a month ago.

Ukraine did not immediately comment on the claim.

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