US officials have said they are investigating a credible but unconfirmed threat of a terror attack, reportedly involving bomb-laden vehicles ahead of this weekend's 10th anniversary of the 9/11 attacks.
"There is specific, credible but unconfirmed threat information," the Department of Homeland Security said in a statement on Thursday.
"We have taken, and will continue to take all steps necessary to mitigate any threats that arise," the department added in a statement.
Law enforcement sources said a manhunt was under way for two or three suspects, the Reuters news agency reported.
US officials said the threat involved New York City and Washington DC; the sites involved in the al-Qaeda attacks a decade ago this Sunday that killed nearly 3,000 people.
The White House confirmed that US President Barack Obama had ordered that counter-terrorism efforts be stepped up.
At a press conference on Thursday, Michael Bloomberg, New York's mayor, said the city was deploying additional police resources around the city to keep residents safe.
"We take all threat reporting very seriously," he said. "We continue to ask the public to remain vigilant as we head into the weekend."
But Bloomberg added: "There is no reason for any of the rest of us to change anything in our daily routines."
Al Jazeera's Cath Turner, reporting from New York, said that authorities had warned of a possible car or truck bomb threat.
"There is going to be more security measures and presence around bridges, tunnels, subways and any other kind of public transport," our correspondent said.
"There are threats every day to New York City but obviously this one was considered significant enough to come out and warn the public," she added.
Raymond Kelly, New York's police commissioner, said he was increasing patrols around the city and stepping up the towing away of illegally parked cars. He said there would be more bomb-sniffing dogs on the streets and the subways with bag inspections at subway stations also increased.
Police armed with heavy weapons had been deployed outside of Manhattan to respond citywide, Kelly said. Extra patrols would check on tunnels, bridges, ferries and landmarks.
US media said three individuals were reported to have entered the country in August and were allegedly seeking to carry out an attack using an explosives-laden vehicle.
Peter King, a congressman for New York and head of the House Committee on Homeland Security, would not confirm any details after legislators were briefed about the situation.
But he told CNN that "very, very specific facts" were known about the threat."
"I can tell you that the administration, all levels of law enforcement, federal, state and local where it's appropriate, are checking out every possible lead, running this to ground, and we've come a long way since September 11."
The announcement came after US defence officials on Wednesday raised the alert level at bases across the United States as "a prudent and precautionary measure".
Sunday's biggest event will be the reading of victims' names at Ground Zero, which will be attended by victims' family members, as well as Obama, George W Bush, his presidential predecessor, and Bloomberg.
Security is the main concern among organisers of the ceremonies remembering those who died and large crowds are expected as the country reflects on a decade of war abroad and fear of further attacks at home.