Russian forces seized control of the Zaporizhzhia nuclear plant in southern Ukraine, Europe's largest, and the surrounding region shortly after the February 24 invasion.

Both sides have traded blame for recent shelling near the plant lying on the frontline, sparking fears of a nuclear disaster.

A 14-strong team of inspectors from the International Atomic Energy Agency arrived at the facility on Thursday to conduct "security and safeguards activities" after a risky journey across the frontline and early-morning shelling of the area.

"It is obvious that the plant and physical integrity of the plant has been violated several times," IAEA head Rafael Grossi told reporters after returning to Ukrainian-controlled territory.

Grossi said part of the IAEA mission will stay at Zaporizhzhia "until Sunday or Monday" to continue the assessment, without specifying their number.

The Argentine described the visit as productive and said he gathered lots of information.

Wearing bright blue flak jackets and helmets, the IAEA team crossed into Russian-held territory, reaching the facility at around 1200 GMT.

After the inspection, in a video released by the Russian RIA Novosti news agency, Grossi said: "We have achieved something very important today and the important thing is the IAEA is staying here."

A dawn shelling attack on the area had forced one of the plant's six reactors to close.

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

It said the plant's emergency protection system kicked in shortly before 5:00 am (0200 GMT), shutting reactor five, with the attack damaging a back-up power supply.

'Stop playing with fire'

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

He warned the consequences of hitting the plant could be "catastrophic", saying "the slightest miscalculation could trigger devastation that we will regret for decades."

After Russian forces seized the plant on March 4, Energoatom shut two reactors, followed by a third after shelling on August 5. With a fourth undergoing repairs, Thursday's incident leaves only one of the six reactors working.

Mardini said it was "encouraging" the IAEA team was inspecting the plant because the stakes were "immense".

On leaving Zaporizhzhia, Grossi said his team would be travelling through areas where "the risks are significant" but had decided to go ahead anyway due to the "very important mission to accomplish".

Shelling and saboteurs

The neighbouring town of Energodar came under sustained attack at dawn, with Russian troops firing "mortars and using automatic weapons and rockets", its mayor Dmytro Orlov said.

But Moscow accused Kyiv of smuggling in up to 60 military "saboteurs" who reached the area near the plant at dawn, prompting Russian troops to take "measures to annihilate the enemy".

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

Kyiv also suspects that Moscow intends to divert power from the plant to the nearby Crimean peninsula, annexed by Russia in 2014 -- a view held by other international figures including British Prime Minister Boris Johnson.

Meanwhile, Ukrainian troops pressed ahead with a counter-offensive in the nearby region of Kherson to retake areas seized by Russia at the start of the invasion.

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

- Back-to-school gunfire soundtrack -

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

In the southern Mykolaiv region, children were back in front of screens for online classes as all school attendance was cancelled due to the ongoing fighting.

On her first day back, nine-year-old Antonina Sidorenko, who lives in a hamlet near the frontline, was doing her online lessons with the distant crackle of gunfire in the background.

"I'm happy to be back at school but I would be even happier if there was no war because I miss my teacher and my friends," she told AFP, saying her best friend had fled to Poland.