At least three people have been killed in the massive explosion that levelled two buildings in New York City Wednesday, officials are now saying.

At least 64 others have been injured and nine occupants of the buildings are missing.

A gas leak is believed to have triggered the large explosion.

“This is a tragedy of the worst kind because there was no indication in time to save people,” New York City Mayor Bill de Blasio said at an earlier news conference.

He said preliminary information suggests the only warning came just 15 minutes before the 9:30 a.m. blast, when a gas leak was reported to utility company ConEdison.

Utility and fire crews arrived just after the explosion.

The collapsed buildings at 1644 and 1646 Park Ave. in East Harlem held apartments, a piano store and a storefront church.

As soon as the fire is out, crews will begin searching through the rubble, de Blasio said.

“I emphasize that those who are missing could well be safe in another location and just not contacted yet or reachable yet,” he said.

De Blasio said the “extended” recovery and clean-up operation will involve 250 firefighters and heavy machinery.

Rescue workers were only able to begin the search for victims amid the rubble Wednesday evening, as it took hours for firefighters to extinguish the fire.

Recovery efforts are being hampered however, due to inclement weather and a sinkhole caused by a water main break, officials said.

The Mount Sinai Hospital treated 23 patients, most of whom experienced minor injuries such as cuts, scrapes, broken bones and inhalation problems, said Dr. Kevin Chason.

One female patient was critically injured with head trauma, and is listed in "stable but critical" condition in the ICU.

Meanwhile, nearby buildings have been evacuated and the Red Cross has set up an emergency shelter at a public school.

“Everybody got scared because the blast was so strong,” George Lizzano, whose office is nearby, told CTV News Channel Wednesday.

“All the buildings (were) moving and shaking like an earthquake.”

Lizzano said he saw people being taken away on stretchers, bleeding and seemingly unresponsive.

TV footage showed heavy smoke billowing from the buildings and firefighters blasting water on the site from extended ladders.

A number of cars parked along the street in front of the buildings were crushed by falling debris. The blast also shattered windows in nearby buildings and storefronts. Police have cordoned off a two-block radius.

The site of the reported explosion is near train tracks. Commuter train service in the area has been temporarily suspended.

The National Transportation Safety Board said it is conducting an investigation at the site. The agency probes pipeline issues as well as transportation disasters.

With files from The Associated Press