The International Atomic Energy Agency (IAEA) has appealed for a halt of the use of force at Ukraine's largest nuclear power plant and warned of "severe danger if any reactors were hit."

The United Nations nuclear watchdog said in a statement early Friday that it was informed by Ukraine that the Zaporizhzhia nuclear power plant, the largest in Europe, had been shelled overnight in the eastern city of Enerhodar. IAEA Director-General Rafael Mariano Grossi "immediately" spoke with Ukrainian Prime Minister Denys Shmyhal as well as the country’s national nuclear regulator and operator about the "serious situation." Grossi is expected to hold a press conference later Friday.

According to IAEA, the Ukrainian regulatory authority said a fire at the site had not affected "essential" equipment and plant personnel were taking mitigatory actions, and that there was no reported change in radiation levels at the plant.

Ukraine's State Emergency Service said the blaze, which occurred in a training building after shelling from Russian forces, was extinguished Friday morning.

The IAEA said it is putting its Incident and Emergency Center (IEC) in "full response mode" due to the situation at the Zaporizhzhia nuclear power plant. The agency continues to closely monitor developments at the facility and remains in constant contact with Ukraine.

Ukrainian President Volodymyr Zelenskyy gave an urgent plea for intervention after officials said Russian troops shelled Ukraine's largest nuclear power plant overnight.

"Russia wants to repeat Chernobyl, but six times worse," Zelenskyy said in a video message. "Europe wake up, tell your politicians: 'Russian troops are shelling a nuclear power plant in Ukraine. Zaporizhzhya nuclear power plant.'"

The president warned there are six power units at the Zaporizhzhia nuclear power plant -- where a fire ignited during the shelling -- and only one power unit exploded at the Chernobyl nuclear power plant, the site of the 1986 nuclear disaster.

"The Russian military must be stopped immediately," Zelenskyy said. "Shout to your politicians. Ukraine has 15 nuclear units. If there is an explosion, it is the end of everything. The end of Europe. This is the evacuation of Europe. Only immediate action by Europe can stop Russian troops. Prevent the death of Europe from the catastrophe at the nuclear power plant."

President Joe Biden spoke with Zelenskyy to receive an update on the fire at the power plant and "joined President Zelenskyy in urging Russia to cease its military activities in the area and allow firefighters and emergency responders to access the site," the White House said in a readout of the call.

British Prime Minister Boris Johnson also spoke to Zelenskyy about the "gravely concerning situation" at the power station and said he would be seeking an emergency U.N. Security Council meeting "in the coming hours," his office said.

A White House official told ABC News its latest information shows no indications of elevated levels of radiation.

The International Atomic Energy Agency, the United Nations nuclear watchdog group, also tweeted that Ukraine's regulatory authority said there hasn't been any change in radiation levels reported at the site in the wake of the blasts.

-ABC News' Uliana Lototska and Molly Nagle

Russian troops are firing at Ukraine's largest nuclear power plant, officials said.

"Russian army is firing from all sides upon Zaporizhzhia NPP, the largest nuclear power plant in Europe," Ukrainian Minister of Foreign Affairs Dmytro Kuleba tweeted early Friday. "Fire has already broke out. If it blows up, it will be 10 times larger than Chornobyl! Russians must IMMEDIATELY cease the fire, allow firefighters, establish a security zone!"

Andriy Tuz, a spokesperson for the power plant, warned in videos posted to Telegram that Russian shelling caused a fire at the plant. He said the first power unit was hit and that rescuers have not yet been able to put out the fire due to the ongoing shelling.

The International Atomic Energy Agency, the United Nations nuclear watchdog group, said only Unit 4 was in operation currently and "radiation levels and fire safety at the NPP is within the norm." The fire broke out at a training building outside the power plant, the IAEA said.

The power plant's livestream captured blasts and a fire at the facility.

The situation at Ukraine's largest nuclear power plant has become "critical," authorities had warned earlier Thursday as Russian troops approached the site.

"The battle is going on in the town of Enerhodar and on the road to the ZNPP (Zaporizhzhia Nuclear Power Plant) site," Ukraine's regulatory authority said in an "urgent letter" to the IAEA.

Ukraine reported that Russian tanks and infantry “broke through the block-post” to the town of Enerhodar, near the power plant, according to IAEA Director General Rafael Mariano Grossi, who called on troops to refrain from use of force.

During an emergency IAEA board of governors meeting this week, Grossi warned that an incident affecting nuclear facilities "could have severe consequences, aggravating human suffering and causing environmental harm" beyond Ukraine's borders.

IAEA tweeted it is aware of reports of shelling at the power plant and is in contact with Ukrainian authorities.

-ABC News' Cindy Smith, Kirit Radia and Bill Hutchinson

Facebook restrictions in Russia are becoming more extensive, according to internet monitoring agency NetBlocks.

The restrictions impact internet providers including Rostelecom and Beeline, according to NetBlocks.

They follow restrictions placed on Twitter and Facebook in recent days that have caused some content to take an extremely long take to load, if at all, on major providers, making the platforms "largely unusable," NetBlocks reported.

Circumvention is possible using VPN services.

The independent Russian news site Meduza also reported Thursday that it is inaccessible for some readers in Russia.

-ABC News' Christine Theodorou and Patrick Reevell