The head of the United Nations nuclear agency said Friday he was pushing for access to the roof of the Zaporizhzhia Nuclear Power Plant in Ukraine after Ukraine and Russia accused each other anew of planning attacks. The plant was seized by Russia, in March 2022, in the first weeks of the war in Ukraine, raising fears of a nuclear accident.

The Russians have granted only limited access to officials from the International Atomic Energy Agency, citing security.

 Wrapping up a four-day visit to Japan, IAEA Director General Rafael Mariano Grossi said his agency was making progress on access to Zaporizhzhia.“It’s like a conversation and I’m pushing to get as much access as possible,” Grossi said, in an interview with The Associated Press in Tokyo, adding that there was “marginal improvement.”

 “I’m optimistic that we are going to be able to go up and see,” Grossi said, referring to the plant’s rooftops.

 Citing intelligence reports, Ukrainian President Volodymyr Zelenskyy had alleged Tuesday night that Russian troops placed “objects resembling explosives” atop several power units to “simulate” an attack as part of a false flag operation. The “foreign objects” were placed on the roof of the plant’s third and fourth power units, the General Staff of Ukraine’s armed forces said.

 The Associated Press reviewed high-resolution satellite imagery of the plant taken Monday and Wednesday.

The photos showed no visible changes to the roofs of the six concrete containment domes covering the reactors at the plant, or nearby buildings.

In Russia, Kremlin spokesman Dmitry Peskov this week called the situation at the nuclear plant, which is Europe’s largest, “quite tense,” and accused Kyiv of planning to sabotage it.Neither Ukraine nor Russia has provided any evidence for its claims of an imminent threat.

Grossi said the IAEA had modeled the possible environmental impact of an explosion or bombing at the plant.

You have enough nuclear material to create quite a disastrous situation in the worst scenario, but then there are different alternatives that could happen,” he said.

The UN atomic watchdog has repeatedly warned of the possibility of a radiation catastrophe like the one at Chernobyl, about 300 miles to the northwest, where a reactor exploded in 1986.

The Zaporizhzhia plant has been shelled numerous times since the war began.

Regular power outages have made it impossible to operate the plant safely, and its six reactors have been shut down to minimise the threat of a disaster.

The IAEA has officials stationed at the Russian-held plant, which is still run by its Ukrainian staff.

Grossi told the AP that the IAEA had gained access to additional parts of the site recently, including the cooling pond and fuel storage areas.

The Ukrainians had said the areas were mined by the Russians, but the IAEA was able to “confirm that they were not, which is important.”

On Wednesday, Grossi said the most recent IAEA inspection of the Zaporizhzhia plant found no mining activities.

So far “nothing happened, which doesn’t mean things won’t happen,” he said. “I am very concerned and we continue to e very concerned.”

He was speaking after a visit to the tsunami-wrecked Fukushima Daiichi nuclear power plant, where equipment has been installed for the planned release of treated radioactive water into the Pacific Ocean.

Japanese regulators on Friday granted TEPCO a permit to release the water. In its final assessment report issued Tuesday, the IAEA endorsed the plan, saying any environmental and health impact would be negligible.

The government and TEPCO, the plant operator, say the water must be removed to prevent any accidental leaks and make room for the plant’s decommissioning, and hope to start releasing the water this summer.

The plan has faced strong opposition from Japanese fishing community worrying about reputational damage, and groups in South Korea and China that have raised concerns.

Grossi, however, said the controlled discharge of the water treated to levels way stricter than international safety standards and further diluted by massive amount of seawater will be almost undetectable and its impact won’t cross borders.

A massive earthquake and tsunami on March 11, 2011, destroyed the Fukushima Daiichi plant’s cooling systems, causing three reactors to melt and contaminating their cooling water, which has leaked continuously.

The water is collected, treated and stored in about 1,000 tanks, which will reach their capacity in early 2024. Keeping the massive radioactive water on the Fukushima Daiichi is a “festering” problem and leaving it unresolved is a “serious setback” for Japan and for the region, Grossi said as he ensured the safety of the controlled release of treated and diluted water into the Pacific Ocean.

The release is crucial for nuclear safety, and key to regaining confidence and trust for the viable economy, he said. “(Release) is a huge benefit for Japan, for the prefecture and the region and for the larger, the wider area. For me it is self-evident benefit for all.”

Grossi was to leave Tokyo later Friday to head to South Korea, where he will provide an explanation of the safety of the Fukushima water release plan.