A mysterious natural gas-like odor hit Manhattan Monday morning, causing building evacuations from Rockefeller Center to the city's West Side, but the city's mayor said it was not dangerous.

The New York Fire Department began getting calls about the odor around 9 a.m., said spokesman Tim Hinchey. The city's 911 system has been flooded with calls about the smell.

"It may just be an unpleasant smell, but at this point we don't know anymore than that," New York Mayor Michael Bloomberg said at a press conference Monday. "One thing we are very confident of is it is not dangerous, but how long and what the sources are, we just don't know."

Natural gas is a colorless, odorless gas. The chemical mercaptan is added to make it smell.

"What you're smelling is mercaptan, which may or may not come from treated natural gas or may be a leak of that particular chemical on its own," Bloomberg said. "At this point, we don't know."

Bloomberg said the city's air sensors have not reported any elevated or "unusual" levels of natural gas.

A small leak did occur Monday morning at Bleecker Street and Sixth Avenue, but it doesn't appear to be the cause of the smell, Bloomberg said.

A U.S. counterterrorism official said nothing indicates terrorism.

New York FOX affiliate WNYW-TV reported that New Jersey's PSE&G is investigating whether the smell might have originated with a gas leak in Jersey City, N.J.

That city's mayor's spokeswoman Maria Pignataro, a spokeswoman for Jersey City Mayor Jerramiah Healy, said officials there were told the odor was due to a gas leak in Manhattan's Chelsea neighborhood, just north of Greenwich Village.

Commuter train traffic was suspended for a time by the Port Authority of New York-New Jersey between Jersey City and 33rd Street in Manhattan, officials said.

The Metropolitan Transportation Authority temporarily evacuated a control tower at West 4th Street and also closed the 23rd Street subway station briefly.

Utility officials with Consolidated Edison had no immediate comment.

"We have been in touch with Con Ed but they're not reporting any drop in pressure that would be the kind of thing you would expect if there was a large break to their gas system," Bloomberg.

In August, seven people were treated at hospitals after a gaseous smell in the boroughs of Queens and Staten Island.

FOXNews.com's Sara Bonisteel and The Associated Press contributed to this report.