Authorities in New York are attempting to identify a persistent gas smell across a large part of lower Manhattan.
Manhattan was shrouded in low cloud on a misty winter's day
Morning commuters in Manhattan reported a strong gas-like smell, but Mayor Michael Bloomberg said he was confident the odour was "not dangerous".
A stretch of the commuter train service between New York and New Jersey was temporarily halted, and some Manhattan office buildings were evacuated.
The US Department of Homeland Security ruled out any link to terrorism.
Mr Bloomberg said there were no indications of high levels of natural gas on New York's energy network.
"It may just be an unpleasant smell, but at this point we do not know any more than that," he told a news conference, adding that investigations were continuing.
"The one thing we are confident about is, it is not dangerous," he said.
The city quickly appeared to return to normal, although there was no early agreement about what the smell was.
Across the Hudson River in New Jersey, officials said the source of the smell was a gas leak originating in the Chelsea district of Manhattan, but that could not immediately be confirmed.
Gas supplier Consolidated Edison said it fielded about 700 calls from people worried about the smell, which appeared to have died out by early afternoon.
Office worker Brandon Atkins told the BBC that the fumes seeped into his Manhattan building.
"Most people evacuated although the smell was worse on the street," he said.
"My wife in Jersey City told me she could also smell it there so it seems to cover quite a large territory."
New York residents contacted the BBC, describing the smell as a mix of natural gas and burnt rubber, or saying it was a strong methane odour.
Gas supplier Consolidated Edison, which pipes gas into much of New York City, said it was investigating the reports, but told US TV network MSNBC that there was "no abnormal flow" of gas in Manhattan.
There was a previous report of a gas-like smell in August last year in parts of Queens and Staten Island, the Associated Press news agency said.
Natural gas is mainly composed of methane, but has no smell itself.
A substance called Mercaptan is added to the compound to give it a smell to allow its early detection in the event of a leak.