The head of the United Nations' nuclear watchdog said the situation at the Zaporizhzhia nuclear power plant remains "extremely complex and challenging."

International Atomic Energy Agency Director General Rafael Grossi, who just returned from leading a team of over a dozen experts sent to inspect the besieged plant, said during a briefing Friday that the physical integrity of the building "has been violated not once, but several times," and that the integrity of the power supply to the plant is a major concern.

Grossi said he asked to talk to the Russian military at the plant and "they were not available" and noted that his team was not allowed to access the power plant's "crisis center." He implied that the Russians are using it for some other purpose but that it does "not have an influence on the operation of the plant."

There are currently six IAEA experts at the plant, he said. Four of them are set to leave next week and two will stay for the foreseeable future.

-ABC News' Tom Soufi Burridge

A major pipeline that supplies natural gas from Russia to Europe will be indefinitely shut off, Russian energy corporation Gazprom said Friday.

Gas supplies via the Nord Stream pipeline have been suspended since Wednesday for maintenance of its only operative gas compressor unit, Gazprom said. The company was planning to restart the flow of natural gas on Saturday barring any issues.

An oil leakage was detected during the scheduled maintenance and gas transmission "has been fully shut down until the operational defects in the equipment are eliminated," Gazprom said. The company did not give a timeline on the restart.

The shutdown marks Russia's latest supply cutback to Europe following its invasion of Ukraine, as Europe faces an energy crisis.

Several experts with the United Nations' nuclear watchdog have left the Zaporizhzhia power plant, according to Russian news agency Interfax.

Four out of the nine vehicles that arrived earlier Thursday as part of the International Atomic Energy Agency convoy left at 2:15 p.m. local time after about four hours at the plant, according to an Interfax reporter on the ground at the Ukrainian facility.

IAEA Director General Rafael Grossi, who is leading the mission, was among those to leave, while some experts remained behind and will stay at the plant, according to Interfax.

Ukraine's nuclear regulator, Energoatom, also confirmed Thursday that the majority of the IAEA mission has left the power plant, including Grossi, and that five experts stayed behind to unload equipment brought to the plant.

The IAEA team is expected to remain at the plant through Sept. 3, Energoatom said in an update posted to Telegram.

In a video statement posted to Twitter, Grossi said he completed a first tour of "key areas" at the plant on Thursday.

"Of course there's a lot more to do," he said. "My team is staying on, and more importantly and most importantly, we are establishing a continued presence ... from the IAEA here."

-ABC News' Natalia Shumskaia and Fidel Pavlenko

Federal agents searched the New York City apartment of Russian billionaire Viktor Vekselberg on Thursday, law enforcement sources told ABC News.

The oligarch's Park Avenue apartment was searched by federal agents with the FBI and Homeland Security Investigations, the main investigative arm of the U.S. Department of Homeland Security, the sources said.

An address in East Hampton associated with Vekselberg is also being searched as part of court-authorized activity involving the Justice Department's KleptoCapture task force, according to sources.

The task force has been seizing assets of Russian businessmen associated with Russian President Vladimir Putin over suspected violations of U.S. sanctions following the invasion of Ukraine.

The task force is trying to find yachts, airplanes and other moveable property before the oligarchs can transport them to jurisdictions where it might be more difficult for U.S. authorities to investigate.

In April, Spanish authorities seized Vekselberg's $90 million yacht in the port of Palma de Mallorca at the request of the Justice Department.

Vekselberg was among the oligarchs previously sanctioned by the U.S. after Russia invaded Crimea in 2018.

-ABC News' Aaron Katersky

U.S. President Joe Biden made an unannounced visit to war-torn Ukraine on Monday, arriving in Kyiv as Washington signals its ongoing support ahead of the one-year anniversary of Russia's invasion.

Biden's visit came ahead of a planned meeting with NATO allies in Poland. He is expected to give a speech at the Royal Castle Arcades in Warsaw on Tuesday evening to offer an appraisal of international support during the first year of the war and to address "how we will continue to stand with the people of Ukraine for as long as it takes," White House spokesperson Karine Jean-Pierre said in a statement earlier this month.

Biden also plans to meet in Poland with leaders of the Bucharest Nine, a group of eastern NATO allies formed in 2015 in response to Russia's annexation of Ukraine's Crimean Peninsula.

Ukrainian President Volodymyr Zelenskyy made a dramatic visit to the United States in December, his first known international trip since the Russian invasion began in February 2022. Zelenskyy met with Biden at the White House in Washington, D.C., before addressing members of U.S. Congress on Capitol Hill.