Five people are confirmed dead and at least 12 injured in a powerful blast that rocked a gas-fired power plant in Middletown, Connecticut State of the United States Saturday, the mayor's office said.

"Police in Middletown said there was an explosion with mass casualties just before 11:30 a.m. (16:30 GMT) at the Kleen Energy plant on River Road," the local CBS station WFSB reported on its website.

"At least 250 people were injured with at least two people confirmed dead," said WFSB.

CNN quoted Middletown police sources as saying that two people have been confirmed dead, but the casualties are expected to rise since polices are still searching for people.

Middletown is 23 miles south of Hartford, capital of the state of Connecticut.

The facility where explosion occurred was a 620 megawatt gas- fired power plant. It was still under construction and was slated to go online later this year, according to CNN.

The explosion took place when workers switched on the plant's energy generating systems during a test, local media Hartford Courant quoted witnesses as saying.

Al Santostefano, the deputy fire marshal in Middletown, told Hartford Courant that at least 50 construction workers were on the site when the explosion occurred.

"It's possible that there might be people trapped in the rubble, " he said. "There was a lot of steel, from what I could see."

As many as 20 ambulances swarmed the site, and helicopters were airlifting victims to nearby hospitals.

People who live around 20 miles away reported having felt the explosion, which many mistook for an earthquake.