Europe's largest nuclear facility fell to Russian forces shortly after the invasion of Ukraine in February last year and Kyiv and Moscow have since accused each other of planning an incident at the site.

On July 23 International Atomic Energy Agency experts "saw some mines located in a buffer zone between the site's internal and external perimeter barriers," agency chief Rafael Grossi said in a statement on Monday.

The statement did not say how many mines the team had seen.

The devices were in "restricted areas" that operating plant personnel cannot access, Grossi said, adding the IAEA's initial assessment was that any detonation "should not affect the site's nuclear safety and security systems".

Laying explosives at the site was "inconsistent with the IAEA safety standards and nuclear security guidance" and created additional psychological pressure on staff, he added.

Last week the IAEA said its experts had carried out inspections at the plant, without "observing" the presence of any mines, although they had not been given access to the rooftops of the reactor buildings.

The IAEA had still not been given access to the roofs of the reactor buildings and their turbine halls, its latest statement said.

After falling into Russian hands, Europe's biggest power plant was targeted by gunfire and has been severed from the grid several times, raising fears of a major nuclear accident.

The six reactor units, which before the war produced around a fifth of Ukraine's electricity, have been shut down for months.