Engineers on Sunday were probing the cause of a fire at a New York state nuclear power plant after an electrical transformer failed, forcing the shutdown of one of its two reactors, a spokesman said.

There was no release of radioactivity and no threat to worker or public safety, the Indian Point Energy Center said in a statement.

The fire broke out in one area of the Indian Point power plant in Buchanan, only about 35 miles (55 kilometers) north of New York city, after an electrical transformer failed Saturday evening, the plant's spokesman Jerry Nappi said.

The reactor has been operational since 1976 and, according to the US Nuclear Regulatory Commission, underwent a series of unplanned shutdowns in 2006-2007, prompting additional inspections.

In April 2007, a transformer failure and fire was classified as a low-level emergency but no radiation was released, the NRC said.

In Saturday's incident, there was no damage to the nuclear reactor itself, which automatically shut down, and the fire did not occur in the part of the plant where the reactor is housed.

"The transformer is an electric piece of equipment, it has not really much to do with the nuclear side of the plant, so it doesn't affect radiological safety," Nappi told AFP.

He said electrical engineers were on site Sunday to look into what caused the failure.

They "will investigate what occurred, understand why it occurred, and then replace the transformer with a new one," he said.

"There will be a more formal investigation that will occur," he added.

That investigation will be led by engineers from Entergy Corporation, which owns the plant.

- 'No minor' nuclear incidents -

New York Governor Andrew Cuomo visited the plant and cautiously reassured the pubic there about the scale of the incident.

"This was a relatively minor situation, but when you are talking about a nuclear power plant, there are no really minor situations," he told journalists.

Environmental groups, notably the Sierra Club and the Natural Resources Defense Council, have for years called for the aging nuclear facility's closure, citing seismic risks and the plant's proximity to the densely populated New York metropolis.

The fire was extinguished in a matter of minutes by automatic sprinklers and on-site fire personnel, Nappi said.

The accident caused oily fluid to leak into the river, and clean-up crews were dispatched Saturday, though it is not clear what environmental impact the leak might have.

Nappi said the liquid could have been from the transformer itself or was possibly just firefighting foam.

"It would be unlikely that there would be enough of that fluid to impact the environment in any consequential way, but we have experts on site and expert contract clean-up crews to mitigate that," he said.

Indian Point produces about 25 percent of the electricity for New York city and Westchester county.

The reactor that was shut down has a capacity of 1,041 megawatts, according to Entergy.