Russian forces are advancing on Mykolaiv, another key city in southern Ukraine, the regional governor warned Friday.

In a video message posted on social media, Mykolaiv Oblast Gov. Vitaliy Kim said Russian troops are moving on Mykolaiv city from two directions and that some have already entered the city limits but are not yet inside in significant numbers.

The city is preparing to defend itself, according to Kim.

"Don't panic," Kim said. "At the moment, the enemy is approaching from two directions but they're not on our streets yet. We're preparing the defenses, so women and children should get home now and the men join the defense lines."

Kim said the Ukrainian military has a large amount of armour in Mykolaiv and urged residents "not to shoot at every vehicle," since some could be Ukrainian.

"No need to shoot at everything that's moving in the city. There's a lot of our armor in the city," he said. "{lease do not shoot inside the city, there's no enemy here yet, but they are approaching."

-ABC News' Patrick Reevell

The U.S. Embassy in Kyiv took to Twitter on Friday to condemn Russia's shelling of Ukraine's Zaporizhzhia nuclear power plant, the largest in Europe.

"It is a war crime to attack a nuclear power plant," the embassy tweeted. "Putin's shelling of Europe's largest nuclear plant takes his reign of terror one step further."

NATO Secretary-General Jens Stoltenberg met with U.S. Secretary of State Antony Blinken and other NATO foreign ministers in Brussels on Friday morning to discuss the response to the ongoing Russian invasion of Ukraine.

During a press conference prior to the meeting at NATO headquarters, Blinken and Stoltenberg condemned Russia's attacks on civilians in Ukraine and expressed concern over the reports of Russian shelling at Ukraine's largest nuclear power plant.

"This just demonstrates the recklessness of this war and the importance of ending it, and the importance of Russia withdrawing all its troops and engage in good faith in diplomatic efforts," Stoltenberg told reporters. "We provide support to Ukraine. At the same time, NATO is not part of the conflict. NATO is a defensive alliance, we don't seek war conflict with Russia."

Blinken emphasized that NATO and the United States "seek no conflict."

"But if conflict comes to us, we're ready for it," he added. "And we will defend every inch of NATO territory."

The head of the United Nations nuclear watchdog said Friday that no radioactive material was released at Ukraine's largest nuclear power plant amid shelling from Russian forces overnight.

The shelling sparked a fire in a training building at the Zaporizhzhia nuclear power plant, the largest in Europe, in the eastern Ukrainian city of Enerhodar. The blaze has since been extinguished, according to Ukraine's State Emergency Service.

Two security employees at the plant were injured during the incident, according to International Atomic Energy Agency (IAEA) Director-General Rafael Mariano Grossi.

"The safety systems at the six reactors were not effected," Grossi said at a press conference in Vienna on Friday morning. "No radioactive material was released."

"We are following the situation very, very closely," he added.

-ABC News' Joe Simonetti

The Russian forces charging toward Kyiv haven't made progress in the last day as they face Ukrainian resistance and low food and gas supply, a senior U.S. defense official told reporters Tuesday.

However, it could be a deliberate pause, the official said. "Part of the stall could be ... as a result of their own self-determined sort of pause in operations -- that they are possibly regrouping, rethinking, reevaluating," the official said.

The U.S. believes Russian forces "have committed now more than 80% of what was their pre-staged combat power," the official added.

The official said some Russian soldiers weren't told they were going into combat. The official said "not all of them were apparently fully trained and prepared."

The strong Ukrainian resistance has also hurt morale, according to the official.

Russia has now launched more than 400 missiles on Ukraine, the official said. The U.S. believes Russia has launchers that could be used for thermobaric weapons, but cannot confirm their use, the official said.

Russian forces are making the most progress in the south. Russians are attacking Kherson in south Ukraine, which "appears very much to be contested city at this point," the official said.

Russians are also approaching Mariupol in southeastern Ukraine, and while they haven't yet entered the city, "they are close enough now that they could attack Mariupol with long range fires," the official said.

Two towns on the path to Mariupol are believed to be occupied by the Russians, according to the official.

The U.S. believes the Russians hope to move north out of Mariupol up to the heavily-contested city of Kharkiv. The official said they believe Russian forces are trying to encircle Kharkiv.

The U.S. official noted that they've seen "certain risk-averse behavior by the Russian military" over the last week.

"Take the amphibious assault, for instance. They put those troops ashore a good 70 kilometers away from Mariupol because they knew Mariupol was going to be defended and they could put them ashore in an uncontested environment. And they still haven't reached Mariupol," the official said.

"They are not necessarily willing to take high risks with their own aircraft and their own pilots," the official said.

"And of course we're seeing that on the ground -- the fairly slow and steady progress that they have made, and you guys are seeing it for yourselves on the ground where ... units are surrendering, sometimes without a fight."

-ABC News' Matt Seyler