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 Sunday, the mayor said.

Police seal off the road to the blast site at the Kleen Energy plant in Middletown, Connecticut, the United States, Feb. 7, 2010. (Xinhua/Gu Xinrong)

"We know that 12 individuals have been injured. Five individuals are known to have lost their lives," Mayor Al Santostefano told a news conference.

He, however, said the exact number of casualties is not yet available as it is not known how many workers were at the site when the explosion occurred at the Kleen Energy Plant at about 11: 30 local time.

Earlier reports said there were 50 workers at the site at the time of the blast.

"Multiple contractors were on site - adding to the confusion on the number of employees actually working on site," the mayor's office said earlier in a statement.

The power plant is not yet fully on line and the explosion occurred during a testing procedure, the statement said.