Initially, more people were missing, but they have since been accounted for, leaving two still missing, sources said.
The explosion yesterday occurred in a vibrant section of the East Villageone building at Second Avenue and Seventh Street, leveling three buildings, scattering debris and prompting street closures.
All buildings and businesses in the vicinity of the explosion site have been evacuated, with 144 residential units currently under vacate order. Some 50 firefighters are still trying to extinguish an active fire.
"The initial impact appears to have been caused by plumping and gas work that was occurring inside one of the buildings," the mayor said, adding that fire department personnel have been "battling heavy fire conditions" in the "complex and difficult operation".
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Images of the scene showed smoke engulfing the area with residents of the building making desperate attempts to escape the fire and flee to safety.
"Our thoughts and our prayers are with every one of them and their families. And of course, we are praying that no other individuals are found injured, and that there are no fatalities," de Blasio said.
Shameem Noor, a cashier at a restaurant nearby, said he heard a "big sound" and people fall on the street.
"People were running and screaming... There's a big fire on the roof and black smoke," he said.
Mark Sydorak, a 70-year-old accountant who works nearby, said he rushed outside when he heard a loud explosion.
Fire Commissioner Daniel Nigro said the explosion blew the front of one of the buildings and initially it was "extremely dangerous" to search the buildings for any victims.
Authorities have set up a temporary shelter and relocation centre in a neighbouring school. The buildings were privately-owned with one of them housing a private company.
The incident comes a year after a huge explosion caused by a gas leak destroyed two buildings in Harlem neighbourhood, killing two people and injuring 18 others.