New York Mayor Michael Bloomberg has confirmed there is a "specific, credible" terror threat coinciding with the 10th anniversary on Sunday of the attacks on 11 September in 2001.
Mr Bloomberg says the threat has not been corroborated, but security will be boosted at bridges, tunnels and on public transport.
Speaking at a joint news conference, Police Commissioner Raymond Kelly said spot checks in the New York subway would be more frequent.
"There is no doubt that New York is safer now than it was 10 years ago, but there are no guarantees. We don't know what we don't know and we are doing everything that I believe we can do to protect the city, but it is a dangerous world," he added.
The Department of Homeland Security says the information it has received is unconfirmed, but all steps necessary are being taken to mitigate any threats that arise.
Homeland Security is asking the American people to remain vigilant as the nation heads into the weekend.
Reuters news agency says the threat appears to focus on Washington DC and New York City.
The NBC television network says talks are under way on whether to raise the national alert status.
