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The New York City Fire Department says it occurred around 10:30 a.m. at the site of a former Mobil gas station on 14th Street and Tenth Avenue in Chelsea.

Officials say the workers were trying to remove several underground gas tanks when the explosion occurred.

"As they were cutting that steel, probably a spark from that saw as they were cutting caused the tank to heat, which caused the vapors inside the tank to explode," said FDNY Batallion Chief Mike Myers.

"The work was being done pursuant to a safety plan. The tank had been flushed out more than a week ago. They had been monitoring for the build-up of vapors and other things," said Ken Fischer, an attorney for the property's owner.

The two workers were taken to Bellevue Hospital with serious but non life-threatening injuries.

Two nearby buildings were evacuated shortly after the blast which could be heard blocks way.

Witnesses at the scene described a very loud, possibly underground explosion with a fireball rising up but then quickly going out.

"I got close to the window, and you could see the ball of fire, smoke and fire coming up. Obviously you don't know what's going on so your reaction is run, or see. The safety guy started taking us out of the building. But it was a literal scary moment," said one worker who was nearby.

"People want to see in the window but nobody let the people, everybody says 'Go out, go out.' So everybody go out," said another worker.

A portion of the nearby Highline was briefly shut down but has since reopened.

Work at the construction site has been stopped as the investigation continues.