Rafael Grossi, director general of the International Atomic Energy Agency, said he had received reports that the soldiers were close to the Zaporizhzhia station in eastern Ukraine.
The six reactors there were safe for the moment and Russian troops had not tried to enter the facility, a statement said.
But Grossi said he was following developments in Ukraine "very closely and with grave concern".
He added: "It is extremely important that the nuclear power plants are not put at risk in any way.
"An accident involving the nuclear facilities in Ukraine could have severe consequences for public health and the environment."
A meeting of the IAEA's governors is scheduled for Wednesday at its Vienna headquarters to discuss the risks that the conflict poses to Ukraine's nuclear facilities.
Ukraine has four nuclear power stations currently in operation providing about half the country's electricity.
It is also the site of the world's worst nuclear disaster, the 1986 Chernobyl fire. Russian troops captured the site on Thursday.
On Sunday, the agency said Ukraine's nuclear authorities had reported that missiles had hit a radioactive waste disposal facility in Kyiv, although there was no damage to the building and no radioactive leak.