A team from the UN nuclear agency has arrived at the site of Europe's largest nuclear plant to inspect security conditions, the agency and Ukraine's nuclear energy operator said on Thursday, amid fighting between Russian and Ukrainian forces that prompted the shutdown of one reactor.

Enerhoatom said the International Atomic Energy Agency team, which is being led by director Rafael Grossi, arrived at the Zaporizhzhia nuclear plant that has been in the thick of recent fighting between Ukrainian and Russian forces more than six months after Russian President Vladimir Putin ordered his troops to invade Ukraine.

The company said earlier on Thursday that Russian mortar shelling had led to the shutdown of one of its reactors by its emergency protection system and had damaged a backup power supply line used for in-house needs.

One of the plant's reactors that wasn't operating was switched to diesel generators, Enerhoatom said.

Ukraine and Russia trade blame for heavy shelling that had temporarily delayed the team's movements toward the sprawling plant on the left bank of Dnieper River in an area controlled by Russian forces.

The fighting underscored the risks faced by the team of UN inspectors that was hoping to assess the safety of the plant.

“There has been increased military activity, including this morning until very recently," Grossi said, adding that after being briefed by the Ukrainian military he decided to get moving despite the inherent risks.

“But weighing the pros and cons and having come so far, we are not stopping.”

Grossi, who personally negotiated with Ukrainian authorities to allow the team's passage, noted earlier that the risks were “very, very high” in the so-called grey zone between Ukrainian and Russian positions, but “we consider that we have the minimum conditions to move."

Russia's Defence Ministry said some the Ukrainian shells landed 400 metres away from the plant's Reactor No 1 Ukrainian officials, meanwhile, accused Russian forces of shelling Enerhodar and a corridor that the IAEA team was set to go through, before it was finally able to clear checkpoints and reach the site.

Neither side's version of events could immediately be independently verified.

Russian Foreign Minister Sergey Lavrov said Moscow expects “impartiality” from the team.

“We are taking all the necessary measures to ensure that the plant is secure, that it functions safely and that the mission accomplishes all of its plans there,” he said.

The Zaporizhzhia plant has been occupied by Russian forces but run by Ukrainian engineers since the early days of the 6-month-old war.

Ukraine alleges Russia is using the plant as a shield, storing weapons there and launching attacks from around it, while Moscow accuses Ukraine of recklessly firing on the area.

Fighting in early March caused a brief fire at its training complex, and in recent days, the plant was temporarily knocked offline because of damage, heightening fears of a radiation leak or a reactor meltdown.

Officials have begun distributing anti-radiation iodine tablets to nearby residents.

“We have a very important mission to accomplish,” Grossi said, adding that “we are going to start immediately an assessment of the security and the safety situation at the plant.”

The Russian Defence Ministry said Ukrainian forces unleashed an artillery barrage of the area and then sent a group of up to 60 scouts to try to seize control of the nuclear plant.

It said the Ukrainian troops arrived in seven speedboats, landing three kilometers northeast of the plant on the left bank of the Dnieper River and tried to seize it.

The ministry said Russian forces “took steps to destroy the enemy,” engaging warplanes. Russia's military said its forces also destroyed two barges carrying Ukrainian troops who attempted to land near the plant.

“The provocation by the Kyiv regime is intended to derail the arrival of the IAEA's group at the Zaporizhzhia nuclear power plant,” a ministry statement said.

The fighting came as war-torn Ukraine endeavoured on Thursday to start the new academic year in the best way possible, with civilian areas still under threat of artillery fire and other weaponry — and children still among the victims.

Just over half of schools in Ukraine were reopening to in-person education despite the risks.