Emergency crews clearing debris from the site of two New York City apartment buildings levelled in a gas explosion said they expected to reach an underground area on Sunday where natural gas lines could reveal what caused the deadly blast.

Eight people were killed when the five-story buildings exploded in the East Harlem neighbourhood early Wednesday, just moments after a resident called 911 to complain of the smell of gas.

Firefighters and other emergency workers have been working around the clock to clear debris from the site, the New York City Fire Department said Sunday.

"We are still operating as a search-and-recovery and we are still clearing debris," spokesman Khalid Baylor said. No more people are believed to be trapped in the wreckage of the building.

On Saturday, crews had removed the majority of the rubble from the flattened buildings and were able to make their first push underground.

NYC Explosion

Firefighters continue to investigate and remove debris from an explosion in Harlem, Wednesday, March 12 in New York. A gas leak triggered an explosion that shattered windows a block away, rained debris onto elevated commuter railroad tracks close by, cast a plume of smoke over the skyline and sent people running into the streets. (Bebeto Matthews/Associated Press)

Investigators want to reach the front of the basements where gas pipes and meters are located to determine whether a natural gas leak led to the explosion and, if so, what caused the breach.

"We'll have it all cleared so we can get to the basement to start our investigation," Fire Commissioner Salvatore Cassano said Saturday.

7 victims identified

The National Transportation Safety Board which investigates pipeline accidents, said on Friday tests conducted hours after the explosion indicated high concentrations of natural gas underground.

Consolidated Edison Inc, the utility provider for New York City, said it had workers on hand to assist both the recovery mission and the investigation.

Five women and three men were killed and dozens of other people were injured when the buildings collapsed under the force of the explosion. The structures housed 15 apartments, a ground-floor church and a piano store.

On Saturday, workers removing tons of debris unearthed an intact Bible belonging to the Spanish Christian Church, which lost five members in the explosion.

The pastor of the church, Rev. Thomas Perez, was so overwhelmed to see the Bible he suffered heart palpitations and was taken to hospital for observation, Cassano told reporters.

Seven of the eight victims have been identified.

They are Carmen Tanco, 67, Alexis Salas, 22, Griselde Camacho, 44, Rosaura Hernandez, 21, Andreas Panagopoulos, 43, George Ameado, 44 and Rosaura Barrios, 44.