Clinton says Al-Qaeda behind 9/11 10th anniversary threat

Speaking after she attended 9/11 commemorations at the New York Stock Exchange, Clinton said the threat - reportedly involving 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," Clinton said.

"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, 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.

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.

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.

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.