"The electricity that it produces rightly belongs to Ukraine and any attempt to disconnect the plant from the Ukrainian power grid and redirect to occupied areas is unacceptable," State Department spokesman Vedant Patel told reporters.

"No country should turn a nuclear power plant into an active war zone and we oppose any Russian efforts to weaponize or divert energy from the plant," he said.

Ukraine earlier said that Zaporizhzhia -- the largest nuclear facility in Europe -- for the first time was severed form its national grid after fighting around the complex.

Patel said that the United States had no indication of abnormal radiation levels and applauded the "courage and selflessness" of Ukrainian workers at the plant.

President Joe Biden, in a telephone call with his Ukrainian counterpart Volodymyr Zelensky, called for Russia to return full control of the plant and let in UN nuclear inspectors, the White House said.

Bonnie Jenkins, the under secretary of state for arms control, said Moscow -- which invaded Ukraine in February -- bore responsibility for the plant's safety.

"Their actions at the Zaporizhzhia nuclear plant have created a serious risk of a nuclear incident -- a dangerous radiation release -- that could threaten not only the people and environment of Ukraine but also affect neighboring countries and the entire international community," Jenkins told reporters.

"The risk of radiation release could be all but eliminated if Russia returns control of the Zaporizhzhia nuclear power plant back to Ukraine and fully withdraws from Ukraine's sovereign territory," she said.