A huge explosion has rocked a power plant in the US state of Connecticut, killing at least five people and injuring 12. Middletown Mayor Sebastian Giuliano said it was caused by a gas explosion.

There are reports of people trapped under the remains of a plant building, and a rescue operation is under way. Search experts and specialist dog teams were at the site combing through the wreckage, the mayor said.

Speaking at a news conference on Sunday evening, Mr Giuliano said that the identities of the dead had not yet been released as some of the victim's families had not yet been informed. Mr Giuliano said the plant, which was only 95% complete and due to open fully in the summer, was undergoing a series of gas tests when the explosion occurred.

Between 100 and 200 people worked on the massive site, and were employed by multiple contracting companies.

"How many people were here today - that's the number they can't really nail down. They were purging gas lines all night long, most people were evacuated from the building when they ran the tests," Mr Giuliano said.

The plant is located to the south of the town, on the Connecticut River.

At least 100 firefighters were sent to the scene and it had taken them an hour to contain the fire caused by the blast, he said. Officials say there is no further danger to the public, and have not evacuated the area. The plant, however, remains cordoned off.

The nearby Middlesex Hospital told the Associated Press it had received 11 victims, while the hospital in the state capital, Hartford, has treated three others.