The US secretary of state has warned that al-Qaeda is behind a credible threat to Americans in the run-up to the anniversary of 9/11.
Hillary Clinton said there had been a "specific, credible but unconfirmed report" of an attempt to target New York and Washington DC.
President Barack Obama is to visit New York and Washington on Sunday as the US marks 10 years since al-Qaeda attacked.
Nearly 3,000 people died in New York, Washington and Pennsylvania in 2001.
'Eyes and ears of vigilance'
Speaking after she attended 9/11 commemorations at the New York Stock Exchange, Mrs Clinton said the threat - which is reported to involve bomb attacks using truck or cars - was the work of the radical group that attacked the US 10 years ago.
"Al-Qaeda again is seeking to harm Americans and in particular to target New York and Washington," Mrs Clinton said on Friday.
"This should not surprise any of us. It is a continuing reminder of the stakes in our struggle against violent extremism, no matter who propagates it, no matter where it comes from, no matter who its targets might be."
She said the decision to go public with the threat was "intended to enlist the millions and millions of New Yorkers and Americans to be the eyes and ears of vigilance", adding that the threat was being taken very seriously by state and federal authorities.
Counter-terrorism officials were particularly concerned by the threat because documents seized during the raid on Osama bin Laden's Pakistan compound in May showed al-Qaeda was considering strikes to coincide with the anniversary, Mrs Clinton said.
The former New York senator said Americans should not change their plans, but instead be alert.
"You have to go on with life," she said, adding, "This city is resilient."
Events commemorating the anniversary of the attacks are scheduled throughout the weekend.
President Obama will visit the World Trade Center site, the Pentagon, and a memorial ceremony in Shanksville, Pennsylvania - where the fourth hijacked plane, United 93, came down.
He will be joined in New York by former President George W Bush.
Mr Obama, who was briefed on the new alert before addressing Congress on Thursday, called on the US to "redouble" anti-terror efforts.
Speaking on Thursday night, New York Mayor Michael Bloomberg said the threat was "uncorroborated", but security would be boosted at bridges, tunnels and on public transport.
Mr Bloomberg travelled on the New York subway on Friday morning in a symbolic gesture aimed at showing the city is fully prepared for a terror threat.
Police Commissioner Raymond Kelly said spot checks in the New York subway would be more frequent in coming days.
The NYPD has also tightened security on New York's roads, bridges and tunnels.
Checkpoints have been set up across Manhattan where police are scanning for radiation and stolen license plates, causing serious traffic congestion around the city.
Earlier, Mr Kelly told the BBC he had concerns about a new threat.
In the US capital, police chief Cathy Lanier said the public should expect increased security measures and more stopped vehicles, the Washington Post reported.
Police shifts have been extended indefinitely and members of the public are being asked to report abandoned or suspicious vehicles, the Post said.
Reports suggest that intelligence gathered in Pakistan points to a possible car or truck bomb attack against New York or Washington.
US TV network ABC News reported on Thursday evening that three individuals - one a US citizen - entered the US in August aiming to carry out a terror attack.
Unconfirmed reports suggest that US authorities are searching for the trio. But it is unclear whether this information was linked to the threat detailed by US officials.
Officials have said the identities of the three people are unknown.
Reuters news agency reported US officials as saying the threat could be linked to al-Qaeda leader Ayman al-Zawahri - Osama Bin Laden's deputy, who took over leadership of the group after Bin Laden's death.
Speaking at Thursday's news conference alongside Mr Bloomberg and Mr Kelly, the FBI Assistant Director in Charge of the New York office, Janice Fedarcyk, confirmed the threat.
"Al-Qaeda has shown an interest in important dates and anniversaries. In this instance it is accurate that there is credible, specific but unconfirmed information," she said.
However, the terror alert level has not changed.
"The thing we are all most worried about is what they call a 'lone ranger,' a lone actor, not some extremely complicated plan like it took to take down the World Trade towers." Vice-President Joe Biden said, speaking on US TV on Friday morning.
An anonymous counter-terrorism official told the Associated Press that authorities had been investigating the intelligence since it was received late on Wednesday.
Mr Kelly told the BBC earlier that the city's police would be on high alert during the weekend.
"We are worried specifically about something happening on the anniversary of 9/11 because we saw on some of Bin Laden's materials that there was discussion about the 10-year anniversary, the 10-year memorial," he said.
"There is no doubt that New York is safer now than it was 10 years ago, but there are no guarantees. We are doing everything that I believe we can do to protect the city, but it is a dangerous world."
Mr Bloomberg told reporters that New York police would be "deploying additional resources... some of which you will notice and some of which you will not".