U.S. authorities were investigating a gas-like odour that covered much of Manhattan and parts of New Jersey on Monday, but New York Mayor Michael Bloomberg said he was confident it was not dangerous.

The mysterious odour prompted officials to evacuate some buildings and suspend a section of the city's commuter train service.

Workers who left their offices after smelling a mysterious gas-like odour stand outside the Time Life Building in New York on Monday.
(Adam Rountree/Associated Press)

Reports of the smell started coming in around 9 a.m. ET as firefighters raced around searching for the source.

Bloomberg said city air sensors were not showing high levels of natural gas.

"It may just be an unpleasant smell, but at this point we do not know any more than that. The one thing we are confident about is, it is not dangerous," he said.

Bloomberg said officials had detected a small gas leak in Greenwich Village, but it wasn't large enough to account for the pervasive odour, the mayor said.

Crews from Consolidated Edison, which provides electricity to the city, were investigating, but they had found no unusual changes in the gas flow within their transmission system, said spokesman Chris Olert.

"If there was a big leak, we would see a change in the gas flow," Olert said.

According to a report by Reuters, the smell had mostly dissipated by noon.

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

U.S. Department of Homeland Security spokesman Russ Knocke told reporters that there was no indication that the smell was part of an extremist threat against the city. But he said officials were closely monitoring the situation.

In August, a gaseous smell hit parts of Queens and Staten Island, sending seven people to hospital. Its cause was never discovered, despite an environmental probe.

With files from the Associated Press