A strong foul smell in Manhattan and neighbouring New Jersey, which generated hundreds of calls to emergency services on Monday, had all but dissipated after about five hours but its cause and source were still not known.

In Jersey City, two people, complaining of breathing problem, were hospitalised in the only known incident of adverse effect.

The smell was so strong that many in high rise office and residential buildings thought it was coming from the gas leak in their own buildings leading to a number of evacuations.

But New York Mayor Michael Bloomberg and the Homeland Security Department moved quickly to assure people that it was not dangerous and that terrorism was not involved.

Mobile air sensors, officials said, showed no higher level of natural gas.

At best, it was unpalatable smell, they said.

Similar smell was reported from parts of New Jersey across the river and it was unclear whether it came from the same source or from a different one.

First reports had said that it might be coming from a gas leak but the Consolidated Edison, which supplies electricity and gas to Manhattan, said their crews had noticed no unusual change in pressure in its transmission lines to account for any leak.

It would have to be a major leak for the smell to affect such a large area.

Natural gas has no smell and additives like mercaptan are added so that nay leak can be detected. Very small quantity of additives is required and hence one theory was it might be leak of such a substance.

The Department of Homeland Security said there is no indication of terror.

"There is no indication at this time of a terrorism connection. Nor is there any credible intelligence to suggest an imminent threat to the Homeland or to New York City at this time," said a Homeland Security spokesman.

Officials said low clouds might have prevented the smell from dissipating and it might have been carried by the wind over several blocks in Manhattan.

Some commuter trains were halted temporarily as officials investigated but were later resumed.