A huge explosion ripped through a US power plant yesterday being built in Connecticut amid reports up to 50 people may have died, emergency officials said, as a rescue operation swung into place.

The blast at the gas-fired plant in Middletown, home to 40,000 people on the Connecticut River, sent flames and black smoke billowing into the sky and shook houses several kilometres away, witnesses said.

As helicopters, ambulances and fire trucks rushed to the scene and a massive search and rescue operation was launched, officials were reluctant to say how many might have died, but a large number of fatalities was feared.

"The reports vary from a few, several to possibly as many as 50 dead," Brian Albert from the Middlesex hospital, which was treating several of those injured in the blast at the Kleen Enery plant, told AFP.

"They are in the process of search and rescue," Albert said, adding that the Middlesex was treating six patients and a seventh had been transferred to the nearby Hartford hospital, which confirmed it was also handling injured.

CNN reported at least two fatalities but officials would not immediately confirm the number of deaths or injured at the plant in the northeastern US state of Connecticut.

One witness told the local Hartford Courant newspaper: "There are bodies everywhere." Other witnesses suggested many victims could still lie buried in the rubble.

"There was a massive explosion, there are multiple injuries and possible fatalities," Middletown police spokesman George Yepes said.

Dozens of emergency personnel swarmed around the sprawling energy complex.

The Hartford Courant reported that 20 ambulances were at the scene and said helicopters were airlifting some of the victims to nearby hospitals.

A second spokesman at the Middlesex said an emergency command centre was being set up there to deal with the incident. "We don't know what we've got at the moment," she added, referring to the state of the injured.

Pictures of the site showed a convoy of ambulances driving into the facility, from which a huge, black plume of smoke rose into the sky.

A local resident told the Hartford Courant that the explosion took place during a test of the plant's power generating systems.

The 620-megawatt Kleen Energy plant, said to be one of the largest power facilities to be built in New England for many years, was still in the process of being built.

The gas-fired energy production plant is located on the outskirts of Middletown on the edge of the Connecticut river, but close to some residential housing.

A company called Energy In-vestors Funds recently acquired 80 percent of the Kleen Energy plant, which had been due to go online sometime in 2010.