New York City emergency responders work at the site of an explosion and collapse of two buildings in the Harlem section of New York, March 12, 2014. [Photo/Agencies]

NEW YORK - A gas leak triggered an earthshaking explosion that flattened two apartment buildings on Wednesday, killing at least six people, injuring more than 60 and leaving nine missing. A tenant said residents had complained repeatedly in recent weeks about "unbearable" gas smells.

The blast struck a building at 116th Street and Park Avenue in Upper Manhattan at about 9:30 am (1330 GMT), blowing out windows in surrounding buildings and sending debris crashing down onto nearby streets.

Two five-story residential buildings next to each other collapsed, with one essentially gone from the blast. Preliminary investigation suggested that the only warning before the blast was a call about a gas leak.

Utility company Con Edison said it received the call from a resident in an adjacent building about 15 minutes before the explosion. The company said it sent two workers to the scene after the call, but they arrived two minutes after the explosion.

In a media briefing more than two hours after the explosion, Mayor Bill de Blasio said that the explosion was caused by a gas leak. A heavy fire ensued as well.

"This is a tragedy of the worst kind because there was no indication in time to save people," he told reporters at the scene.

There will be a search those who are missing through the rubble as soon as the fire is put out, de Blasio said.

Earlier Wednesday, local officials said two people were killed in the accident and more than 12 people remained missing, and at least 22 were injured, including two in critical condition.