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

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."

A high-stakes mission from the United Nations' nuclear watchdog reached a Russian-controlled power plant in Ukraine on Thursday afternoon amid reports of heavy fighting there.

The International Atomic Energy Agency has long sought access to the Zaporizhzhia nuclear power plant, Europe's largest, since invading Russian troops overran the site and the surrounding town of Enerhodar in southeastern Ukraine in early March. The Ukrainian workers have been left in place to keep the plant operating, as it supplies electricity across the war-torn country, but the site is now on the frontline between Russian-occupied and Ukrainian-controlled territory. Moscow and Kyiv have traded accusations of shelling at or near the plant in recent days and weeks, fueling fears that the conflict could spark a radiation disaster.

IAEA Rafael Grossi, who is leading a team of over a dozen experts sent to inspect the besieged plant, said earlier Thursday that they were "aware" of the high risk posed by the "increased military activity in the area" between Russian and Ukrainian forces.

"There has been increased military activity, including this morning, until very recently, a few minutes ago. I have been briefed by the Ukrainian regional military commander here about that and the inherent risks," Grossi told reporters as he and his team left their hotel in the city of Zaporizhzhia, north of Enerhodar, across the Dnipro River.

"But, weighing the pros and cons, and having come so far, we are not stopping," he added. "We are moving now."

A few hours later, the IAEA announced via Twitter that its "Support and Assistance Mission ... has just arrived at Zaporizhzhya Nuclear Power Plant to conduct indispensable nuclear safety and security and safeguards activities."

A long-awaited expert mission from the United Nations' nuclear watchdog arrived in the southeastern Ukrainian city of Zaporizhzhia on Wednesday.

The International Atomic Energy Agency's team will travel to the Zaporizhzhia nuclear power plant near the town of Enerhodar on Thursday for the first time.

IAEA Director General Rafael Grossi, who is leading the mission, told reporters during a press briefing in Zaporizhzhia on Wednesday that the aim is for his team to establish a permanent presence at the Russian-occupied plant and that the initial phase would take "days."

When asked if it was possible to demilitarize the site, Grossi said it was "a matter of political will" and that his mission is to preserve Europe's biggest nuclear power plant. He admitted it was "not a risk-free mission" and underlined that his team would be operating in Ukrainian sovereign territory but in cooperation with Russian forces.

Asked if he thought Russian troops would really give his team full access, Grossi told reporters the IAEA was on a "technical mission" and that he was confident his team could work "on both sides."

-ABC News' Tom Soufi Burridge