The U.N. atomic watchdog said on Sunday that it needs wider access around the Zaporizhzhia nuclear plant to check "a significant discrepancy" in water level data at the breached Kakhovka dam used for cooling the plant's reactors.

International Atomic Energy Agency head Rafael Grossi, who is to visit the plant this week, said that measurements the agency received from the inlet of the plant showed that the dam's water levels were stable for about a day over the weekend.

"However, the height is reportedly continuing to fall elsewhere in the huge reservoir, causing a possible difference of about two metres," Grossi said in a statement.

"The height of the water level is a key parameter for the continued operability of the water pumps."

The destruction of the Kakhovka hydropower dam in southern Ukraine last week has flooded towns downstream and forced thousands of people from their homes.

Both the Kakhovka hydropower dam and the Zaporizhzhia nuclear plant have been occupied by Russia since the early days of its invasion in February 2022.

The water from the reservoir is used to cool the facility's six reactors and spent fuel storage, the IAEA said.

"It is possible that this discrepancy in the measured levels is caused by an isolated body of water separated from the larger body of the reservoir," Gross said in the statement. "But we will only be able to know when we gain access to the thermal power plant."

Grossi said the thermal power plant "plays a key role for the safety and security of the nuclear power plant a few kilometres away," hence the need for access and independent assessment.

The agency has said earlier that the Zaporizhzhia plant can fall back on other water sources when the reservoir's water is no longer available, including a large cooling pond above the reservoir with several months' worth of water.