As the 14-strong team from the International Atomic Energy Agency (IAEA) left for the Zaporizhzhia nuclear plant, Ukraine said Russian troops had shelled the area, forcing the closure of one of its six reactors.

Energoatom, Ukraine's nuclear agency, said it was "the second time in 10 days" that shelling had forced the closure of a reactor.

The area around the plant -- Europe's largest nuclear facility -- has suffered repeated shelling, with both sides accusing the other of responsibility, sparking global concern over the risk of an accident.

"It is high time to stop playing with fire and instead take concrete measures to protect this facility... from any military operations," ICRC chief Robert Mardini told reporters in Kyiv.

"The slightest miscalculation could trigger devastation that we will regret for decades."

Energoatom said the plant's emergency protection system kicked in shortly before 5:00 am (0200 GMT) "due to another (Russian) mortar shelling" and that "operating power unit five was shut down".

But the backup power supply "was damaged" in the attack, it said.

After Russian forces seized the plant on March 4, Energoatom shut two of the reactors, followed by a third after shelling on August 5.

With a fourth in repairs, Thursday's incident means only one of the six is functioning.

- 'The stakes are immense' -

Mardini said it was "encouraging" that the IAEA team was en route to inspect the plant because the stakes were "immense".

"When hazardous sites become battlegrounds, the consequences for millions of people and the environment can be catastrophic and last many years," he said.

On leaving Zaporizhzhia, IAEA chief Rafael Grossi said his team had been updated about the shelling but would press on anyway.

"We are not stopping," he vowed, despite being aware that in crossing the frontline into Russian-held territory, there was a security "grey area... where the risks are significant".

"We have to proceed with this. We have a very important mission to accomplish."

By early afternoon, the team had crossed into the grey zone after waiting for "several hours at a checkpoint" for permission to cross, Ukraine's Interfax news agency reported, saying "gunfire could be heard" along the road.

- Shelling, saboteurs and back-to-school -

Earlier, Energodar Mayor Dmytro Orlov said his town had come under sustained attack at dawn on Thursday, saying Russian troops had fired "mortars and used automatic weapons and rockets".

Energodar town is located next to the plant on the southern banks of the Dnipro River.

But Moscow accused Kyiv of smuggling in up to 60 military "saboteurs", saying they reached the area near the plant just after dawn and that Russian troops had taken "measures to annihilate the enemy".

Grossi on Wednesday said the IAEA would seek to establish a "permanent presence" at the plant to avoid a nuclear disaster at the facility which is located on the frontline of the fighting.

"My mission is... to prevent a nuclear accident and preserve the largest nuclear power plant in Europe," he said.

Ukraine has accused Russia of deploying hundreds of soldiers and storing ammunition at the plant.

Both Moscow and Kyiv have accused each other of staging "provocations" aimed at disrupting the work of the IAEA mission.

Meanwhile, Ukrainian troops pressed ahead with a counteroffensive in the nearby southern region of Kherson that began on Monday to retake areas seized by Russian forces at the start of the invasion.

In its morning update, the presidency said "heavy explosions continued for the last 24 hours" across Kherson region, while five people had been killed and 12 others wounded in fighting in the eastern Donetsk region.

Despite the conflict, now in its seventh month, September 1 marked the start of a new school year for children across Ukraine.

Figures from Ukraine's education ministry show 2,199 educational institutions have been damaged in the fighting, with 225 of them completely destroyed.

Just over half of the 23,000 schools surveyed by the ministry are equipped with bomb shelters, meaning they can physically reopen, while those without will only offer online learning.

But in Kharkiv, Ukraine's second city, all learning will be online due to constant Russian shelling, its mayor said last month, with a British charity charging Thursday that dozens of its schools had been "targeted".

An investigation by the Centre for Information Resilience found 41 institutions had been "partially or completely destroyed" in the northeastern city, with researchers finding the shelling "was targeted, rather than a by-product of indiscriminate attacks on civilian infrastructure".

Elsewhere, the foreign ministry in London said a British medic volunteering in Ukraine had died in the fighting on August 24 but gave no further details on the circumstances of his death.