Elba Cedeno holds a sign remembering victim Cathy Smith during ceremonies marking the 10th anniversary of the 9/11 attacks on the World Trade Center, in New York Sept 11, 2011. [Photo/Agencies]

NEW YORK - Americans on Sunday remembered the horror of Sept 11, 2001, and the nearly 3,000 people who died in the hijacked plane attacks as authorities worked to ensure the emotional 10th anniversary was peaceful.

Law enforcement authorities in New York and Washington were on high alert against what was described as a "credible but unconfirmed" threat of an al-Qaida plot to attack the United States again a decade after the toppling of the World Trade Center's twin towers by hijacked airliners. Security was especially tight in Manhattan, where police set up vehicle checks on city streets as well as bridges and tunnels coming into the city.

President Barack Obama and former president George W Bush, who was president at the time of the attacks, were scheduled to be among the dignitaries at the Ground Zero site in New York.

They were set to join victims' families to hear the reading of the names of those who died on Sept 11. Bells will toll across the city.

In the Sept 11 attacks, 19 men from the Islamic militant group al-Qaida hijacked airliners and crashed them into the World Trade Center, the Pentagon outside Washington and a field in Shanksville, Pennsylvania. Just weeks after the attacks, US forces invaded Afghanistan to topple that country's Taliban rulers who had harbored the al-Qaida leaders responsible for the Sept 11 attacks.

In Afghanistan, NATO-led forces said on Sunday that on the eve of the 10th anniversary a suicide bomber driving a truck of firewood attacked a NATO base in central Afghanistan in an operation for which the Taliban later claimed responsibility.

NATO said the Saturday afternoon attack killed two Afghan civilians and injured 77 NATO troops, but did not state the nationality of the troops. It said those injuries were not life-threateaning.

To mark the 10th anniversary, Obama was set to visit all three attack sites.

"Thanks to the tireless efforts of our military personnel and our intelligence, law enforcement and homeland security professionals, there should be no doubt: today, America is stronger and al-Qaida is on the path to defeat," Obama said in his weekly radio and Internet address.

US forces killed al-Qaida leader Osama bin Laden in Pakistan in May.