An explosion rocked the Chelsea neighborhood of Manhattan and injured at least 25 people on Saturday (September 17), the New York City Fire Department said, prompting emergency personnel to swarm one of the most bustling areas of the city on a cool autumn evening.

A law enforcement source said an initial investigation suggested the explosion occurred in a dumpster but the cause was still undetermined.

Another official close to the investigation told Reuters that investigators were considering the possibility that the blast was accidentally triggered by construction explosives.

But CNN reported that law enforcement sources believe an improvised explosive device caused the blast.

President Barack Obama, who was attending a congressional dinner in Washington, "has been apprised of the explosion in New York City, the cause of which remains under investigation," a White House official said. "The president will be updated as additional information becomes available," the official added.

A U.S. official said the Joint Terrorism Task Force, an interagency group of federal, state and local officials, was responding to the blast, suggesting authorities have not ruled out the possibility of a terror attack.

The task force is also investigating a pipe bomb that exploded earlier on Saturday in a plastic trash can along the route of a charity road race in the New Jersey beach town of Seaside Park. No injuries were reported in that blast, which authorities believe was a deliberate act.