Ukraine and Russia accused each other Wednesday of planning to attack one of the world's largest nuclear power plants, which is located in southeastern Ukraine and occupied by Russian troops, but neither side provided evidence to support their claims of an imminent threat.

The Zaporizhzhia Nuclear Power Plant has been a focus of concern since Moscow's forces took control of it and its staff in the early stages of the war. Over the last year, the U.N.'s atomic watchdog repeatedly expressed alarm over the possibility of a radiation catastrophe like the one at Chernobyl after a reactor exploded in 1986.

While Russia and Ukraine regularly traded blame over shelling near the plant that caused power outages, Ukraine has alleged more recently that Moscow might try to cause a deliberate leak in an attempt to derail Kyiv's ongoing counteroffensive in the surrounding Zaporizhzhia region.

Citing the latest intelligence reports, Ukrainian President Volodymyr Zelenskyy alleged Tuesday night that Russian troops had placed "objects resembling explosives" on top of several power units to "simulate" an attack as part of a false flag operation.

Also read: Ukraine’s president tells other countries to act before Russia attacks nuclear plant

The "foreign objects" were placed on the roof of the plant's third and fourth power units, according to a statement from the General Staff of Ukraine's armed forces. "Their detonation should not damage power units but may create a picture of shelling from Ukraine," the statement said.

The International Atomic Energy Agency has officials stationed at the Russian-held plant, which is still run by its Ukrainian staff. IAEA Director General Rafael Mariano Grossi said his agency's most recent inspection of the plant found no mining activities, "but we remain extremely alert."

"As you know, there is a lot of combat, I have been there a few weeks ago, and there is contact there very close to the plant, so we cannot relax," Grossi said during a visit to Japan.

In Russia, Kremlin spokesman Dmitry Peskov raised the specter of a potentially "catastrophic" provocation by the Ukrainian army at the nuclear plant, which is Europe's largest but has its six reactors shut down. It still needs power and qualified staff to operate crucial cooling systems and other safety features.

"The situation is quite tense. There is a great threat of sabotage by the Kyiv regime, which can be catastrophic in its consequences," Peskov said in response to a reporter's question about the plant. He also claimed that the Kremlin was pursuing "all measures" to counter the alleged Ukrainian threat.

Also read: Nuclear watchdog growingly worried over Ukraine plant safety

Grossi said he was aware of both Kyiv's and Moscow's claims and reiterated that "nuclear power plants should never, under any circumstances, be attacked."

"A nuclear power plant should not be used as a military base," he said.

Peskov's comments came after Renat Karchaa, an advisor to Russian state nuclear company Rosenergoatom, said there was "no basis" for Zelenskyy's claims of a plot to simulate an explosion.

"Why would we need explosives there? This is nonsense, (aimed at) maintaining tension around the Zaporizhzhia Nuclear Power Plant," Karchaa said.

Russian media on Tuesday cited Karchaa as saying that Ukraine's military planned to strike the plant early Wednesday with ammunition laced with nuclear waste. As of Wednesday afternoon, there was no indication of such an attack.

Last week, Ukrainian emergency workers held a drill to prepare for a potential release of radiation from the plant. According to the country's emergency services, in case of a nuclear disaster at the plant, approximately 300,000 people would be evacuated from the areas closest to the facility.

Ukrainian officials have said the shut-down reactors are protected by thick concrete containment domes.

Also read: Ukraine: Russia put rocket launchers at nuclear power plant

The International Institute for Strategic Studies said last week that "a Russian attack on Zaporizhzhia would probably not lead to the widespread dispersal of significant amounts of radiation" due to precautionary steps taken by the International Atomic Energy Agency.

However, the think tank noted in an assessment that wind might blow some amount of radiation toward Russia.