US President Barack Obama said the 11 September victims would be remembered "no matter how many years pass".
His words came as Americans marked the 11th anniversary of the attacks in which nearly 3,000 people were killed.
Two passenger jets brought down the Twin Towers of New York City's World Trade Center, another hit the Pentagon outside Washington and a fourth crashed in a field in Pennsylvania when passengers aboard that flight fought back against the hijackers.
Mr Obama, speaking at the Pentagon where 184 people were killed, told victims' families that the whole country shares their loss.
"Eleven times we have paused in remembrance and reflection, in unity and in purpose," he said. "This is never an easy day, but it is especially difficult for all of you, the families of nearly 3,000 innocents who lost their lives."
"But no matter how many years pass, no matter how many times we come together on this hallowed ground, know this: That you will never be alone, your loved ones will never be forgotten. They will endure in the hearts of our nation because through their sacrifice they helped us make the America we are today, an America that has emerged even stronger."
Speaking under clear blue skies that recalled the crisp morning of 11 September 2001, Mr Obama said America's fight is not with Islam but with al-Qaeda, the group responsible for the attacks, and its allies.
This is a line he has used several times since taking office promising to mend ties with the Muslim world.
"I've always said our fight is with al-Qaeda and its affiliates, not with Islam or any other religion," he said. "This country was built as a beacon of freedom and tolerance."
At Ground Zero in New York where the towers once stood, the annual reading of the list of 2,983 people killed at the three sites began at 8.39am (1.39pm Irish time).
The first names were read by Patricia Abbott, wife of Alan Jay Richman, who died at the trade centre, and by Allison Adams, wife of Patrick Adams, who also died in the trade centre's collapse.
Moments of silence were observed at 8.46am (1.46pm Irish time), 9.03am, 9.37am and 10.03am, the times of impact for the four planes, and again at 9.59am and 10.28am, the times that the north tower and then the south tower fell.
9/11 health package expanded to cover cancer treatments
The US government has added 14 categories of cancer to the list of illnesses linked to the 11 September.
The National Institute for Occupational Safety and Health announced last night that responders and survivors exposed to toxic compounds from the wreckage will be covered for cancer treatment under the Zadroga 9/11 Health and Compensation Act.
The act, which also covers the 9/11 attacks on the Pentagon, was signed into law by President Obama on 2 January 2011.
The decision addresses concerns over the rising health toll for emergency workers in the wake of the attacks.
Illnesses related to the 11 September attacks have caused an estimated 1,000 deaths.
Last week, the New York City Fire Department etched nine more names into a memorial wall honouring firefighters who died from illnesses after their work at Ground Zero, bringing the total to 64.
Cancers to be covered include lung and colorectal, breast and bladder, leukaemia, melanoma and all childhood cancers.
The programme had already covered respiratory diseases such as asthma and pulmonary fibrosis, mental disorders including depression and post-traumatic stress disorder as well as musculoskeletal conditions.
However researchers have known that responders and survivors, including local business owners and residents, were exposed to a complex mixture of chemical agents, including human carcinogens.
That mix included combustion products from 20,000 gallons of jet fuel, 100,000 tonnes of organic debris, and 100,000 gallons of heating and diesel oil.