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

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 (NRC) said that 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.

Entergy Corp, the company that operates the facility, also said the plant was stable and there was no danger to the public or to employees.

Several emergency calls earlier reported a loud noise at the plant, which is located on the east bank of the Hudson River, a New York State police spokesman said. He said 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 continues to operate, spokesman Jerry Nappi said.

He said there was no information yet as to what caused the transformer failure.

The fire was put out by the sprinkler system at the transformer and on-site personnel, he said.

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."

Witnesses took to Twitter to report hearing an explosion followed by large plumes of gray and black smoke billowing from the plant. "It was a huge black ball of smoke and alarms went off immediately. We pulled over," tweeted Gustavus Gricius, who was driving by when he heard the explosion.

About 30 minutes later, Gricius tweeted that sirens had stopped and the smoke was clearing.

"I was a mile away from Indian Point when the transformer explosion occurred. Yikes..." said one Twitter user, Kevin Daly.

Photographs taken across the river and posted on Twitter at showed a plume of charcoal gray smoke trailing from the plant.

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 plant, whose origin dates back to the 1960s, has long been controversial because of its proximity to the United States' largest city. It has 1,050 employees, according to Entergy.

Indian Point is one of 99 nuclear power plants licenced to operate in the United States and which generate about 20 percent of U.S. electricity use, according to the U.S. Nuclear Regulatory Commission website.

Large transformer explosions or fires are unusual but not unheard of, with rarely more than one or two a year occurring. While they can be shocking to witness, recent incidents have caused minimal disruption at the facilities.