"Fighting near a nuclear plant is dangerous," White House spokeswoman Karine Jean-Pierre told reporters aboard Air Force One during a flight to Kentucky, where President Joe Biden is to tour flood-damaged areas.

"And we continue to call on Russia to cease all military operations at or near Ukrainian nuclear facilities and return full control to Ukraine," Jean-Pierre said.

Zaporizhzhia -- Europe's largest atomic power complex -- was occupied by Russia early in its invasion and recent fighting there has raised fears of a nuclear accident.

Jean-Pierre said the United States is continuing to "closely monitor" the situation at the facility and radiation sensors have "thankfully" not shown any indications of an increase or abnormal radiation levels.

"We are also aware of the reports of mistreatment of the (plant) staff and we applaud the Ukrainian authorities and operators for their commitment to nuclear safety and security under trying circumstances," she said.

The White House spokeswoman said the United States supports the efforts of the UN's nuclear watchdog, the International Atomic Energy Agency, to assist Ukraine with nuclear safety and security measures.

Kyiv called on Monday for the establishment of a demilitarised zone around the nuclear power station in eastern Ukraine.

In recent days, it has been the scene of strikes that have damaged several structures, forcing the shutdown of a reactor.

"What needs to be done is to remove occupying forces from the station and to create a de-militarised zone on the territory of the station," said Petro Kotin, president of Ukraine's nuclear energy company, Energoatom.

The recent fighting at the plant has prompted the IAEA to warn of "the very real risk of a nuclear disaster."

The Kremlin on Monday accused Ukrainian forces of firing on the Zaporizhzhia plant, warning of potential "catastrophic consequences" for Europe.