A gas-like odour covered much of Manhattan and parts of New Jersey Monday as firefighters worked to discover the cause.

Reports of the mysterious odour started coming in around 9 a.m. ET, said New York fire department spokesperson Tim Hinchey.

New York Mayor Michael Bloomberg said sensors are not showing elevated levels of natural gas and Consolidated Edison, which provides electricity to the city, hasn't reported any anomalies.

"If it is natural gas, we'll identify its source, obviously, and shut it down," he said during a morning news conference.

Bloomberg said officials don't know what is causing the smell, but he said he's confident it is not dangerous.

"The concentrations are not enough to be harmful," he said.

The Jersey City mayor's office told CNN the smell originated around 4th Street and Bleecker Street.

In August, a gaseous smell hit parts of Queens and Staten Island, sending seven people to hospital.