Washington (AFP) - Firms operating nuclear power plants and other energy facilities in the US have been hacked in recent months, the New York Times reported.

It quoted what it called an urgent report issued in late June by the Department of Homeland Security.

The intrusions happened in May and June and hit companies including Wolf Creek Nuclear Operating Corporation, which operates a nuclear power plant in Kansas, the Times said.

It did not say how many firms were targeted, and added the origin of the hack was unknown.

But the Times said an "advanced persistent threat" actor was responsible. The paper said this is the language hacking experts often often use to describe government-backed hackers.

The DHS report carried an urgent "amber warning," the second-highest rating for threat severity, the newspaper said.

The Times quoted Wolf Creek officials as saying no operations systems had been affected.

The corporate network and the internet were separate from the network that runs the plant, these officials were quoted as saying.

The report did not specify if the attacks were aimed at industrial espionage or causing damage.

But the report said the hackers "appeared determined to map out computer networks for future attacks".