Troops seized control of the Chernobyl site -- where radioactive waste is still stored -- on February 24, the first day of the invasion.

"Chernobyl is (an) area where they are beginning to reposition some of their troops -- leaving, walking away from the Chernobyl facility and moving into Belarus," the US official said.

"We think that they are leaving, I can't tell you that they're all gone."

Russian troops also captured Europe's largest nuclear plant at Zaporizhzhia on March 4, sparking alarm when shelling caused a fire at a training facility.

UN atomic watchdog chief Rafael Grossi visited the separate South Ukraine Nuclear Power Plant on Wednesday on his first trip to the country since Russia's invasion raised fears of a nuclear accident.

Grossi has repeatedly warned of the dangers of the conflict -- the first in a country with a vast nuclear estate.

Ukraine has 15 reactors at four active nuclear power plants, as well as stores of nuclear waste, including at Chernobyl -- the site of the world's worst nuclear disaster in 1986.

With shelling continuing overnight, Ukraine and Western powers cast doubt on Russia's pledge on Tuesday to reduce military activity around Chernigiv and the capital Kyiv.