At least five people were killed yesterday when a massive explosion rocked a gas power plant being built in Middletown, Connecticut, US.

A natural gas leak is believed to have caused the blast at the 620MW gas-fired Kleen Energy Systems LLC plant.

Some of the 51 construction workers who were on the site at the time may be trapped, according to local reports.

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, reports Reuters.

The explosion happened just before 11:30am during tests at the plant, which is due to come online in the summer.

The fire took about an hour to extinguish.