Russia-Ukraine updates: Putin suspends key US-Russia nuclear treaty

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

Thursday marked the start of a new school year in Ukraine amid Russia's ongoing war.

For many of Ukraine's 4 million schoolchildren, their last day of school was the day before Russian forces invaded their country on Feb. 24. Since then, thousands of schools across Ukraine have been damaged or destroyed, with less than 60% of schools deemed safe and eligible to reopen by the Ukrainian government, according to the United Nations International Children's Emergency Fund.

At least 379 children have been killed in Ukraine since the war with Russia began, while the whereabouts of 223 others are unknown and another 7,013 were among Ukrainians forcibly transferred to Russia from Russian-occupied areas, according to the Ukrainian Prosecutor General's Office. And as of July 31, an estimated 650,000 Ukrainian children living as refugees in 12 host countries were still not enrolled in national education systems, according to UNICEF.

Despite the continued risk, 51% of schools in Ukraine are reopening to in-person education, with an option to study online if parents prefer, according to The Associated Press.

UNICEF is working with the Ukrainian government to help get the country's schoolchildren back to learning, in classrooms when it is deemed safe, and through online or community-based alternatives if in-person is not possible. Some 760,000 children have received formal or non-formal education since the start of the war. More than 1.7 million children and caregivers have benefited from UNICEF-supported mental health and psychosocial support interventions, the agency said in a press release Thursday.

On the first day of Ukraine's academic year, UNICEF Executive Director Catherine Russell visited a rehabilitated primary school damaged during the early weeks of war. Only 300 students can attend at any one time due to the capacity of the school's bomb shelter, a mere 14% of the school's pre-war capacity, according to UNICEF.

"The new school year should be a time of excitement and promise, as children re-enter the classroom and share stories of their summer with friends and teachers," Russell said in a statement Thursday. “Yet, for four million children in Ukraine, the mood is one of trepidation. Children are returning to schools -- many of which have been damaged during the war -- with stories of destruction, uncertain if their teachers and friends will be there to welcome them. Many parents are hesitating to send their children to school, not knowing if they will be safe."