Five people are dead and 12 are injured after a Sunday morning explosion at a Connecticut power plant under construction.

Emergency workers responded to a gas line explosion at the Kleen Energy Systems plant in Middletown at about 11:30 a.m.

Middletown Mayor Sebastian Giuliano confirmed the fatalities in a news release on the town's website.

Giuliano wrote that it would be difficult to confirm if there are additional casualties because there were a number of different contractors working on site at the time of the explosion.

But Deputy Fire Marshal Al Santostefano told The Associated Press that, while emergency crews planned to continue going through the debris overnight Sunday, no one was believed to be unaccounted for.

An estimated 50 workers were inside the gas-fired plant at the time of the explosion, Santostefano said.

The local hospital said two victims were released, while other patients injured in the blast are being treated for broken bones, abdominal injuries, blunt force trauma and other kinds of injuries consistent with an explosion.

The force of the blast was so powerful that it knocked down part of the building.

"The part that collapsed and came in, they are taking that apart in sections piece-by-piece, very carefully," Santostefano said.

A state police helicopter is using heat imaging to find victims buried under rubble and dogs are also searching.

WTNH-TV reporter Bob Wilson told CTV News Channel that co-workers gathered outside the plant to hear word on their colleagues.

"There's not much left of the building. It's all twisted and gnarled and crumpled, it pancaked down in," he said.

The website for Kleen Energy Systems says parts of the 620 megawatt-generating plant were under construction. It was set to open some time in 2010.

Santostefano said the explosion happened while construction workers were purging gas lines.

The Hartford Courant newspaper reported there were as many as 20 ambulances on scene, with helicopters airlifting victims to local hospitals.

One witness told WTNH that she heard a loud explosion.

"We saw a bright orange flame between two smokestacks on the power plant," she said.

"It shook the house and everybody was scared and the kids started to cry because they didn't know if the house was going to be on fire," she said.

A number of area residents posting comments on local news websites wrote their houses shook and they thought the rumbling was an earthquake.

One user on the website for local newspaper The Middletown Press reported that the blast jolted his home as far away as East Haven, about 45 kilometres north.

The newspaper said flames were shooting out of a gas pipeline at one point, until officials shut off the gas.

Witnesses on scene say black smoke was visible for several kilometres.

One user who left comments on the newspaper's website described the blast as "deafening."

"My house shook and stuff actually fell over around 11:40 a.m. this morning," the user, registered as Josh Scott wrote.

The Middletown Press reported that every fire truck in the city was on scene to douse the flames. Information is slow coming because emergency officials are turning away reporters from the scene due to the potential for hazardous material leaks.