Smoke is seen over the Indian Point nuclear power plant in New York, in this handout photo provided by Gustavus Gricius taken May 9, 2015. [Photo/Agencies]

NEW YORK/WASHINGTON - A nuclear power reactor 40 miles (65 km) north of New York City was shut down on Saturday after a transformer fire, but officials said the Indian Point plant was stable and there was no threat to residents nearby.

People in the area reported an explosion and smoke coming from the plant at Buchanan in New York state. But the US Nuclear Regulatory Commission said the fire had been quickly extinguished.

"These events happen occasionally. They are not unheard of and the plant responded as designed," NRC spokesman Eliot Brenner said in a statement. He added the fire occurred at 5:50 pm (2150 GMT) and was put out 25 minutes later.

The NRC designated the incident an "unusual event," which is the lowest of four categories the agency gives to potential safety or security threats at nuclear plants.

Entergy Corp, which runs the facility and is one of the largest US nuclear power operators, also said the plant was stable and there was no danger to the public or to employees.

Several police units responded to the plant after receiving emergency calls from people who heard an explosion and saw smoke over the facility, which is on the east bank of the Hudson River, said New York State police spokesman Kenneth Bozier. No injuries were reported.

The transformer fire, which triggered the closure of the plant's Unit 3 reactor, was extinguished with no damage to the unit, an Entergy spokesman said. The other Unit 2 reactor continued to operate, spokesman Jerry Nappi said.

He said there was no information yet as to what caused the transformer failure. On Friday, Entergy returned the 1,031-megawatt Unit 3 back to service after shutting it down the previous day to repair a steam leak on the non-nuclear side of the plant.

The fire was put out by the sprinkler system at the transformer and on-site personnel, he said. The transformers are located around 300-400 feet (90-120 meters) away from the reactor.

Emergency sirens in the area did not sound following the incident, Nicholas Zachary, a governing trustee in the village of Buchanan, said in a phone interview.

"I don't foresee any kind of issue," he said. "It's happened before, they'll get it fixed and back and running fairly soon I imagine."

Disruptions to power in the New York City area were unlikely. Power plants often trip off, and the power shortfall is met by other plants on the grid.