The safest outcome will be a controlled shutdown of the Zaporizhia NPP units, a US Pentagon spokesman has said.

"We believe that the safest outcome would be a controlled shutdown of the Zaporizhia nuclear power plant reactors, that this would be the least risky course of action in the near term," the Pentagon spokesman said at a briefing, answering questions about developments in southern Ukraine.

He noted that "we also have U.S. scientists that are monitoring radiation sensor data at the power plant and we have seen no indications of increased or abnormal radiation levels so far."

The American official also said that the United States is in contact with Ukrainian counterparts.

"The Ukrainians told us what you see in the open media, that they launched some kind of offensive. We just don't know its level," he said.

"I don't know, of course, if they have penetrated the first lines of Russian defense, but I think that in the next 24-36 hours we will all have a much better understanding of the level of this offensive – or these offensives in the last few weeks," the Pentagon spokesman said.