MIDDLETOWN, Connecticut (Reuters) - At least five people were killed on Sunday when a massive explosion rocked a gas power plant being built in Middletown, Connecticut, the town's mayor said.

Fire officials said they suspected a natural gas leak caused the blast at the Kleen Energy Systems LLC plant and that some of the 51 construction workers who were on the site may be trapped.

Search and rescue teams with dogs and multiple fire departments were still sifting through the rubble after the explosion and fire, Mayor Sebastian Giuliano told a press conference.

Witnesses said flames shot up and black smoke billowed out when the blast shattered the Sunday morning calm. Some people reported windows blown out and the force was felt as far away as East Haven, a distance of 30 miles.

"The whole house shook. I didn't know what it was, whether it was the house or the water heater or what," said Cornelia Hull, who lives across the Connecticut river in Portland.

Dozens of ambulances from across the region and several helicopters rushed to the scene.

Eleven injured people were taken to Middlesex Hospital in nearby Middletown, said Peg Arico, manager of public relations at the hospital. Two were later discharged and one was transferred to a hospital in the state capital, Hartford.

Those admitted had injuries consistent with impact, including broken bones.

The emergency department at Hartford Hospital said it had received two people injured at the plant.

The explosion happened just before 11:30 a.m. during tests at the plant, a 620-megawatt gas-fired facility that was due to come online in the summer. The fire took about an hour to extinguish.

"It was connected with the gas, the natural gas that was coming into the building. That was part of the involvement," said Al Santostefano, deputy fire marshal in Middletown.

Workers for the construction company, O&G Industries, were purging the gas lines when the explosion occurred, he said.

State emergency official Betsy Hard said local authorities had asked the state for help. Connecticut Governor Jodi Rell was due to visit Middletown and had activated the state's emergency operations center in Hartford, her office said.

The Department of Public Health was providing tents at the scene for medical triage and shelter with temperatures below the freezing mark and a brisk wind blowing.

Publicly available information compiled during the approval process for the Kleen Energy plant said it would operate on natural gas using a combined cycle turbine. Such turbines reuse waste heat produced during the generation process, increasing the plant's efficiency.

Much of the plant's power generation equipment was supplied by Germany's Siemens and was being installed but was not yet operating, Siemens spokeswoman Melanie Forbrick said.

"This was a very tragic accident and our deepest sympathy goes out to all those who have been affected," she said, adding the one Siemens employee working at the site was safe.

Siemens was helping in any way it can, Forbrick said.

Contractor O&G Industries, of Torrington, Connecticut, is overseeing all local, state, and federal permitting, engineering and construction of the project.

(Reporting by Alan Elsner, Toni Clarke, Dan Trotta, Ros Krasny, Doina Chiacu and Bryson Hull; Editing by John O'Callaghan)