|New York police have amassed a display of force across the city in response to the enhanced "terror" threat [Reuters]
Hillary Clinton, the US secretary of state, has said that US intelligence believes al-Qaeda is behind a suspected bomb plot coinciding with the 10th anniversary of the September 11 attacks.
Clinton said on Friday that there were "specific, credible, but unconfirmed reports that al-Qaeda is seeking to harm Americans," particularly in New York and Washington - the sites of the attacks 10 years ago that claimed nearly 3,000 lives.
Clinton's comments came as US authorities stepped up a manhunt for three suspects alleged to be involved in the plot.
Government officials, speaking on the condition of anonymity, said on Friday that at least two of the suspects are believed to be US citizens, the Associated Press news agency reported.
Their primary mission was to explode a car bomb in either New York or Washington, but if that proved impossible, they had been ordered to simply cause as much destruction as they could, one official said. `
A CIA informant who had proven reliable in the past approached intelligence officials overseas to say that the men had been ordered by Ayman al-Zawahri, the new al-Qaeda leader, to mark the 10th anniversary of the 9/11 attacks on Sunday by doing harm on US soil.
The informant said the would-be attackers were of Arab descent and may speak Arabic as well as English, AP reported.
Counter-terrorism officials were looking for certain names associated with the threat, but it was unclear whether the names were real or fake, officials said.
US authorities have been working around the clock to determine whether the threat is accurate, but so far, have been unable to corroborate it.
Meanwhile, New York police have amassed a display of force across the city as security levels have been stepped up in response to the enhanced threat.
Officers armed with automatic weapons were stationed on Friday at city landmarks including Wall Street, Times Square and the 9/11 memorial site where the World Trade Center, destroyed in the 2001 attacks by passenger jets piloted by hijackers, once stood.
"The NYPD has set up checkpoints on the roads in many locations, and there are also random inspections taking place in the subway system, with bag checks set up at major stations," Al Jazeera's Asad Hashim reported from the city's financial district.
US President Barack Obama has called on citizens to come together to mark the anniversary of the 9/11 attacks and to not let the terror threat deter them.
"It's clear for all the world to see that the terrorists who attacked us that September morning are no match for the character of our people, the resilience of our nation, or the endurance of our values,'' Obama said on Saturday in his weekly radio and Internet address.
"We're doing everything in our power to protect our people,'' he said. "And no matter what comes our way, as a resilient nation, we will carry on.''