(AFP) - The chief of the UN atomic watchdog said on Wednesday he was working on a compromise security plan for the Moscow-controlled Zaporizhzhia nuclear plant and warned of increased military activity around it.

There are persistent fears over the safety of the plant in the southern Ukrainian region of Zaporizhzhia, where there has been frequent shelling since Russian troops invaded last year.

During a rare visit to the facility, Europe's largest, the head of the International Atomic Energy Agency, Rafael Grossi, said he was working to find a compromise that would suit both Moscow and Kyiv.

"I am trying to prepare and propose realistic measures that will be approved by all parties," Grossi told reporters during a press tour organised by Moscow at the plant, which is controlled by Russian forces.

"We must avoid catastrophe. I am an optimist and I believe that this is possible," said Grossi, who arrived at the plant in a Russian armoured vehicle, surrounded by soldiers in full combat gear.

But he also warned of increasing military activity around the nuclear plant and expressed hope that Russia and Ukraine would agree on safety principles.

He added that the visit to the plant was "extremely useful."

"The idea is to agree on certain principles, certain commitments, including not to attack the plant," he separately told AFP.

Kyiv and Moscow have accused each other of shelling the plant, increasing fears of a disaster. Russian soldiers stationed at the plant told AFP during Grossi's visit Wednesday that they have been preparing for a possible attack from Kyiv.

Their main task is "to prevent an armed takeover" of the site by Ukrainian "saboteurs", one soldier told reporters.

Ukraine denies any such plans.

The United Nations has called for a demilitarised zone around the site.

- 'Anything can happen' -

Grossi said his team had previously focused on the possibility of establishing a security zone around the plant.

"Now the concept is evolving," he said, with his team focusing on the protection of the plant itself rather than "on territorial aspects which pose certain problems."

During the visit Grossi took a guided tour through the huge facility, which had been fortified. He was followed by around 30 journalists. The Moscow-installed director of the plant, Yury Chernichuk, showed Grossi damage the plant sustained during the hostilities.

The Ukrainian nuclear power operator Energoatom earlier in the day distributed footage of a convoy of civilian and military vehicles marked with the letter Z, a symbol emblazoned on Russian military hardware in Ukraine.

This was Grossi's second visit to Zaporizhzhia since Russia invaded Ukraine last February.

The agency has had a team of experts inside the plant since September 2022, but Grossi has said the situation "is still precarious".

Earlier this week, Grossi met with Ukrainian President Volodymyr Zelensky, who said it was not possible to restore safety at the plant with Russia in control.

Renat Karchaa, an advisor to Russia's nuclear power operator Rosenergoatom said Wednesday ahead of the visit that it was unlikely to bring about major breakthroughs.

"We are far from having any illusions that Grossi's visit will dramatically change anything. For us, this is an ordinary working event," he was cited as saying by Russian news agencies.

- Complete withdrawal -

The plant needs a reliable electricity supply to ensure essential nuclear safety and security functions.

But it has suffered repeated electricity outages during the war, causing alarm in the IAEA and the international community.

The Russian invasion has caused devastation across swathes of the country, and despite more than 13 months of gruelling battles, Ukraine's top diplomat on Tuesday struck a defiant tone.

"Russia has to withdraw from every square meter of Ukrainian territory," Foreign Minister Dmytro Kuleba said in a virtual session ahead of the Summit for Democracy, which US President Joe Biden kicked off on Wednesday.

"There should be no misinterpretation of what the word withdrawal implies."

In Ukraine's eastern Donetsk region, fighting in recent months has concentrated on the eastern city of Bakhmut, with Kyiv saying it is holding out in the urban hub to exhaust Russian troops.

General Mark Milley, the chairman of the US Joint Chiefs of Staff, told a news conference in Washington that Russia has not made "any progress whatsoever" around Bakhmut in the last roughly three weeks.

"It's a slaughter-fest for the Russians," he added.

The key military objective of Russia's invasion is the complete capture of Donetsk, which it already claimed to have annexed last year, even as fighting there is ongoing.

Russian authorities said Wednesday that Ukrainian forces had used US-supplied HIMARS systems to strike Melitopol in the Zaporizhzhia region.

The city controlled by Moscow lies around 65 kilometres from the front line and analysts have said it could be a target for a highly-anticipated Ukrainian counter offensive.

© Agence France-Presse