Washington, Feb 8 (DPA) Two people were killed and more dead and injured were expected after a gas-fired power plant in the north-eastern US state of Connecticut exploded Sunday, media reports said.

The explosion occurred while there were about 100 people inside who were said to be testing some recently-completed upgrades to the facility, the Danbury News-Times reported online.

Dozens of ambulances and two medevac helicopters responded to the scene in the town of Middletown, about 150 km north-east of New York City. An estimated 100 firefighters had brought the flames under control by 1:30 pm, the Hartford Courant reported.

Medical rescue personnel told the Courant that at least four of the 100 in the plant were critically injured, another four were injured, in addition to the two dead at the Kleen Energy Systems plant.

The company could not be reached for comment.

Middletown deputy fire marshal Al Santostefano told the Courant it was 'possible that there might be people trapped in the rubble'. Rescue crews were 'in search and rescue mode' by 1 P.M. Sunday and were trying to see 'if there are any survivors'.

'There are bodies everywhere,' a witness was quoted as saying.

The local television station WQAD said that the blast was so powerful that people felt it 20 km away, many thinking that an earthquake had struck.

The facility appeared to be a gas-fired power plant belonging to the Kleen Energy Systems company. A large amount of gas was believed to be in storage at the site.

The site's main building was levelled by the explosion, early eyewitness reports said.

Television images from a distance showed black smoke rising from the site.

The Danbury News-Times cited one eyewitness as saying the blast created a 'massive ball of fire 40 to 50 feet into the air'.

State police fire and explosive investigators were looking into the cause of the fire.