On Thursday, a U.N. inspection team reached the besieged nuclear power plant in Russian-occupied southern Ukraine, after an artillery barrage forced operators to shut down one reactor and switch another to emergency power, illustrating the huge dangers of combat near a radioactive complex.

In one of the most dangerous and complex missions in the history of the International Atomic Energy Agency, a team of 14 experts intended to assess the damage to the Zaporizhzhia Nuclear Power Plant after weeks of shelling, its ability to operate safely, and other dangers.

By evening, the majority of the team had returned to Ukrainian-controlled territory, leaving behind five men who, according to officials, would remain over the weekend.

After returning from the tour, Rafael Mariano Grossi, director general of the IAEA, remarked, "I was concerned, I am worried, and I will continue to be worried" about the plant's safety. He stated that he had spent almost five hours touring major areas of the plant.

"It is evident that the factory and its physical integrity have been compromised multiple times," he told reporters on the side of the road. "By accident or by design? We lack the information necessary to evaluate that. However, this cannot continue to occur."

The International Atomic Energy Agency, the United Nations' nuclear watchdog, had pleaded for access to the threatened plant for months, and as recently as Wednesday, Russian officials cast doubt on their willingness to cooperate.

The agency's team boarded nine armored S.U.V.s on Thursday morning to make the nerve-wracking journey into an active war zone, across the front lines and through multiple checkpoints, on a drive that was forced to take a detour and lasted several hours longer than planned due to shelling along the intended route.

Thursday provided few hints as to what the specialists had discovered. A spokesperson for the IAEA had previously stated that the team planned to present its findings at its headquarters in Vienna by the end of the week. The organization intends to establish a permanent presence at the facility.

While neither Russia nor Ukraine had agreed to a cease-fire in the region, both had stated they would ensure the mission's safety.

The Ukrainian nuclear power company, Energoatom, reported that mortar shells were fired at the plant on Thursday morning as U.N. specialists departed Ukrainian-held territory for the perilous buffer zone dividing the two forces.