A massive manhunt by security and intelligence agencies was underway in the US for three men believed to be planning major terror attacks in America, as secretary of state warned that cities from New York to Mumbai were still under threat.

The manhunt was launched following what US officials now say is “credible and specific” information that US citizens or people with American travel documents are on a mission to explode a car or vehicle bombs in New York or Washington with orders to create as much destruction as they can.

What is causing alarm in the US agencies is the tip off that at least two of the three men involved in the possible Al Qaeda plot are believed to be American nationals.

Word that the Al Qaeda had dispatched a new attack team came to US agencies in mid week from a CIA informer in Pakistan's troubled northwest.

The team has been dispatched by new Al Qaeda chief Egyptian Aiyman al-Zawahiri, who wants to mark the 10th anniversary of the 9/11 attacks by spectacular strikes on the US soil as a revenge for killing of Osama bin Laden.

The would-be attackers are tipped to be of Arab descent and speaking Arabic and English. Though counter-terrorism officials are looking for some names these have not been made public.

Though the authorities have sounded a high alert in both New York and Washington, pushing in more heavily armed forces to patrol the cities, the citizens remained undoubted by a talk of new terror threat and went about their daily routine.

"We are taking the threat seriously," secretary of state Hillary Clinton said in New York at a function.

Clinton said despite being on a path to defeat since Bin Laden's death, Al Qeada continues to pose a security threat in cities from London to Mumbai.

"Cities such as London and Lahore, Madrid and Mumbai have been attacked since 9/11," Clinton said, adding that the threat still remained.

She said: "State and local authorities are taking necessary steps to address it."

Late Wednesday, the officials received details which they consider specific. Jumping into action, American law enforcement authorities increased security measures at airports, nuclear plants and train stations.

At New York's main Penn station, transit authorities and state police were seen carrying assault rifles and wearing bullet proof vests as they watch crowds of commuters.

Police check posts have sprouted at all major road intersections with sniffer dogs carrying out checks on cars and other vehicles.

White House Press Secretary Jay Carney said President Barack Obama instructed the Homeland Security team to redouble their efforts to ensure that every precaution is being taken, to every avenues being explored to pursue the information that they have about those threats and develop those information.

Carney said some four months ago Obama had ordered his top counter-terrorism official, John Brennan, to run to ground, as they say, every potential lead.

"And as late as Tuesday the President chaired a meeting with his homeland security team, to review those progress, to review those situation so," he said.

But there has been no changes to his plan to attend Sunday's ceremony at ground zero in New York and Shanksville, Pennsylvania. A memorial to the victims of United Airlines Flight 93 who died in that Pennsylvania field will be unveiled in a solemn ceremony.