Citing Ukrainian intelligence, the country's president said Russia was "technically ready" to provoke a localised explosion at the facility.

It comes as Russia launched an overnight drone attack on Kyiv after a 12-day break, a Ukrainian military official said on Sunday, with air defence systems preliminarily destroying all targets on their approach.

"Another enemy attack on Kyiv," Serhiy Popko, a colonel general who heads Kyiv's military administration, said in a Telegram post.

"At this moment, there is no information about possible casualties or damage."

Throughout the war nuclear experts have repeatedly raised concerns about the safety of the Zaporizhzhia plant, which Russia seized control of last March, due to shelling near and around it.

Ukrainian military intelligence previously said Moscow's troops had mined Europe's largest nuclear plant and claimed on Friday that the number of Russian personnel is gradually being reduced.

Mr Zelenskyy said: "There is a serious threat because Russia is technically ready to provoke a local explosion at the station, which could lead to a (radiation) release", in a news conference in Kyiv with Spain's prime minister.

He gave no further details and so far, Moscow has not commented on reports of the apparent drawdown at the plant.

Ukraine's GUR claimed that Russian forces are "gradually leaving the territory of the Zaporizhzhia nuclear power plant".

Among the first reported to have left were three staff from Russian state nuclear firm Rosatom who were "in charge of the Russians' activities".

Special Russian military units guard the facility and a unit of Rosatom runs the plant.

Both sides have accused each other of shelling near the plant, amid ongoing fears about a potential disaster at the facility, which is just 500km from the site of the world's worst nuclear accident, the 1986 Chernobyl disaster.

On Friday, the UN's nuclear watchdog, International Atomic Energy Agency (IAEA), said its experts have "so far found no visible indications of mines or other explosives currently planted" at the plant.

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It said its aware of reports that mines and other explosives have been placed in and around the plant - including mines near the cooling pond.

However, director general Rafael Mariano Grossi added that experts still need additional access to carry out further such checks at the site.

Sky News previously spoke to two workers at the plant who warned the consequences of a catastrophe could cause devastation across much of Europe, Russia and the Mediterranean.

The IAEA said its experts were able to inspect parts of the plant's cooling system, including some sections of the perimeter of the large cooling pond and the isolation gate of the discharge channel of the nearby Zaporizhzhya Thermal Power Plant (ZTPP).

It said both this channel and the cooling pond hold reserves of water that remain available for use by the nuclear plant despite the recent destruction of the downstream Kakhovka Dam.

The biggest nuclear risk at the plant is from overheating nuclear fuel, which could happen if the power that drives the cooling systems is cut or if there is not enough water to supply the cooling systems. Shelling has repeatedly cut power lines.