The shelling by Russia in recent days around the Zaporizhzhia nuclear power plant in Ukraine landed close to two of the facility's nuclear reactor buildings, according to the International Atomic Energy Agency.

Rafael Mariano Grossi, director general of the IAEA, said Sunday that Russian rockets hit two "special buildings" about 100 meters, or less than 330 feet, from the nuclear reactor buildings.

Citing a report from Ukrainian officials, Grossi said the buildings hit in the attack house the facility's water treatment plants, equipment repair shops or waste management facilities.

Grossi said there was damage to some water pipelines at the site but they have since been repaired. He said measurements of radioactivity were within normal range and there was no indication of any hydrogen leakage.

He said the nuclear plant, the largest in Europe, continues to access electricity from an off-site source after the plant temporarily lost connection to its last remaining operational 750 kilovolt external power line on Thursday.

Two reactor units that were disconnected from the electricity grid when power was cut to the plant on Thursday have been re-connected, Grossi said. The other four reactor units that were disconnected before Thursday remained shut down on Sunday.

He said the latest shelling once again underlines the risk of a potential nuclear accident at the Zaporizhzhia plant, which is under the control of Russian forces but being operated by Ukrainian staff.

-ABC News' Christine Theodorou

International Atomic Energy Agency director general Rafael Mariano Grossi said he’s engaged in active consultations with all parties to lead and organize an IAEA mission to the Zaporizhzhia nuclear power plant "hopefully in the next few days.” Grossi said in a statement that Thursday’s sudden loss of external power showed that IAEA's presence is urgently needed.

-ABC News' Alex Faul

The head of Ukraine’s nuclear agency, Petro Kotin, called Thursday’s disconnection of power lines at Zaporizhzhia, Europe’s largest nuclear power plant, a "very serious deterioration… due to Russian shelling on the side of Nikopol on the side of the Dnipro river."

The diesel generator operators were able to kick in during the disconnection, Kotin told ABC News on Friday.

Asked what’s the likelihood this would happen again, Kotin said, "This is one step from the emergency radiation, actually. Diesel is just the last defense, if you actually lose the diesel stuff, you already have radiation, and it's out of the territory of the plant."

He said there'd be 10 days of idle time before a meltdown if the diesel generators are working. If all diesel generators are down, it could be 1.5 hours.

Inspectors could gain access to the site in as soon as five days, he said.

For Russia, the Zaporizhzhia plant is a strategic stronghold as it seeks to control the ground pathway from occupied Donetsk to Crimea and onto Odesa.

Zaporizhzhia supplies Ukraine with around 20% of its energy.

Kotin reiterated that Russia intends to switch power to Russian-controlled areas.

-ABC News' Britt Clennett, Dada Jovanovic and Yuri Zalizniak

All of the reactors at Zaporizhzhia nuclear power plant, where shelling has been ongoing, have been shut down for the first time in the plant’s history, Ukraine's state nuclear regulator Energoatom reported.

Zaporizhzhia -- the largest nuclear power plant in Europe -- has six reactors, two of which are active, according to Ukraine's state nuclear regulator, Energoatom. At 12:12 p.m. local time, the last operating line providing power to the plant was disconnected due to hostilities in the area, and as a result all six reactors were disconnected from the grid for 17 minutes, Energoatom said.

At 12:29 p.m. local time the overhead line was restored and reactors Nos. 5 and 6 start operating again.

At 2:14 p.m. local time, the overhead line was disconnected again, shutting down reactor No. 6 and leaving only reactor No. 5 operating. Work is underway to reconnect No. 6 to the grid.

The International Atomic Energy Agency said the temporary shut down further shows "the urgent need for an IAEA expert mission to travel to the facility."

If external power is lost there's not active circulation of the water that cools the reactor and that could lead to a reactor meltdown. However, the plant "remained connected to a 330 kV line from the nearby thermal power facility that can provide back-up electricity if needed," the IAEA said in a statement. "As a result of the cuts in the 750 kV power line, the ZNPP’s two operating reactor units were disconnected from the electricity grid and their emergency protection systems were triggered, while all safety systems remained operational."

"There was no information immediately available on the direct cause of the power cuts," the IAEA said. "The six-reactor ZNPP normally has four external power lines, but three of them were lost earlier during the conflict. The IAEA remains in close contact with Ukraine and will provide updated information as soon as it becomes available."

Ukrainian President Volodymyr Zelenskyy warned Thursday, "The world must understand what a threat this is: If the diesel generators hadn’t turned on, if the automation and our staff of the plant had not reacted after the blackout, then we would already be forced to overcome the consequences of the radiation accident. Russia has put Ukraine and all Europeans in a situation one step away from a radiation disaster."

He called on the IAEA and other international organizations to act faster, "because every minute the Russian troops stay at the nuclear power plant is a risk of a global radiation disaster."

-ABC News' Christine Theodorou, Fidel Pavlenko and Natalia Shumskaia

At least 15 people were killed and two dozen more are feared trapped in rubble after Russian Uragan rockets slammed into a five-story apartment building in eastern Ukraine's Donetsk region, local officials said on Sunday.

Search-and-rescue workers were combing through the rubble for survivors on Sunday.

The missile strike came as Ukrainian officials reported clashes with Russian troops on the frontline in the eastern and southern Ukraine.

Donetsk Governor Pavlo Kyrylenko said the strike on the apartment building occurred Saturday evening in the town of Chasiv Yar. The regional emergency service said Sunday that the death toll had climbed to 15 and that 24 more people could still be buried under the rubble.

"We ran to the basement, there were three hits, the first somewhere in the kitchen," a local resident, who gave her name as Ludmila, told rescuers as they removed a body wrapped in a white sheet and cleared rubble using a crane as well as their hands. "The second (strike), I do not even remember, there was lightning, we ran towards the second entrance and then straight into the basement. We sat there all night until this morning."

Andriy Yermak, chief of staff to Ukrainian President Volodymyr Zelenskyy, said in a Telegram post that the strike was "another terrorist attack" and that Russia should be designated as a state sponsor of terrorism as a result.

Russian officials claimed its forces struck Ukrainian army hangars storing U.S.-produced M777 howitzers, a type of artillery, near Kostyantynivka in Donetsk region, but, so far, has not claimed responsibility for the missile strike on the residential building in Chasiv Yar.

-ABC News' Christine Theodorou