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

U.S. Secretary of State Antony Blinken crossed the border into Ukraine Saturday after meeting with Ukrainian Foreign Minister Dmytro Kuleba at the Korczowa Border Crossing Point.

"The entire world stand with Ukraine," Blinken said, and Kuleba added that he hopes Ukrainians will see this as a "clear manifestation that we have friends who literally stand by us."

Blinken said the sanctions imposed so far against Russia are "producing very, very concrete results" and that the pressure will "grow" until this "war of choice is brought to an end."

When asked directly about helping Kuleba with more firepower, Blinken said, "the support for Ukraine not only has been unprecedented, not only is going to continue, it's going to increase."

Kuleba said Ukraine "appreciates" sanctions that have been announced in the last week, but that more economic and political pressure and "necessary weapons," would "save many lives in Ukraine... many sufferings will be avoided."

Kuleba also said that they are “satisfied" with already arranged supplies of anti-tank weapons and ammunition, but that “it’s no secret” they still want fighter jets and air defense systems.

He also thanked the U.S. for sending stingers, and said they were used on Saturday.

"Just today we shot down three Russian attack aircrafts, which were bombing our cities with the use of stingers but we need -- so to say -- big air defense systems to ensure the safety of our skies. If we lose the skies, there will be much, much more blood on the ground,"Kuleba said.

In response to questions about NATO’s refusal to impose a no-fly zone over Ukraine, Kuleba said it’s a "sign of weakness," but stayed optimistic that they will change their minds.

"The time will come," he said.

-ABC News' Conor Finnegan and Justin Gomez