Washington: US intelligence agencies and security forces are currently on a massive manhunt for three men believed to be planning massive terror attacks in America to coincide with the 10th anniversary of the 9/11 attacks.

The manhunt was launched following "credible and specific" information that the trio would be carrying out terrorist attacks with explosive laden vehicles in cities like New York and Washington, which has forces authorities to elevate the threat level nationwide.

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Multiple media reports said the threat involved a car-or truck-bomb attack aimed at bridges or tunnels in New York or Washington.

A news channel reported that US law-enforcement officials have been alerted to watch for three men "of Middle Eastern descent," possibly riding in a van.

Several top government officials and lawmakers having access to classified intelligence information said that the information about the terror plot has been received from a credible one source, which has proved valuable in the past, but could not be corroborated yet.

"This is somebody who in the past has been a source of reliable information. But the people being quoted are not that person. The information is specific enough that it deserves to be taken seriously. And I'm very grateful that we are," said Senator Joseph Lieberman.

"Well that's about as specific and general as the evidence or the intelligence is, those two cities (New York and Washington), and I would say that people should be alert.

I don't think anybody should be panicked," he said.

"When an administration comes to a decision about whether to let this information out that there is something that we picked up that suggests that there might be an attack being planned, they've always got to make a judgment about whether to make it public.

 Making it public sometimes jeopardize sources. But in this case, I think it's important because it alerts everybody to be on guard this weekend and be careful," Lieberman said.

"A US official said the source indicated al-Qaeda's new leader Ayman al-Zawahri was involved in planning the plot. But the official also said that many in the intelligence community question that and other aspects of the source's information," another reported.
Secretary of State Hillary Clinton said there is a specific, credible, but unconfirmed threat report.

"We take it seriously, as we always do, and particularly because we learned from the raid on bin Laden's headquarters and going through all of the material that al-Qaeda was still determined to attack around anniversaries and particularly New York and Washington," she said.

"It is a real threat," Vice President Joe Biden said.

White House Press Secretary Jay Carney told a group of foreign journalists that US 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.

Obama extends emergency on terrorist attacks

Meanwhile, US President Barack Obama has extended by one more year the national emergency with regard to certain type of terrorist attacks.

The emergency has been in place in since September 14, 2001.

"Because the terrorist threat continues, the national emergency declared on September 14, 2001, and the powers and authorities adopted to deal with that emergency must continue in effect beyond September 14, 2011," Obama said in his notification.

"Therefore, I am continuing in effect for an additional year the national emergency that was declared on September 14, 2001, with respect to the terrorist threat," he said.