Americans paused today to mark the 11th anniversary of the Sept. 11, 2001, attacks, but some said it's time to move forward after a decade of remembrance.

Hundreds gathered at the World Trade Center site in New York, the Pentagon and Shanksville, Pa., to read the names of the nearly 3,000 victims killed in the worst attack in U.S. history.

"Our country is safer, and our people are resilient," President Barack Obama said in a ceremony on the White House's south lawn. He and Michelle Obama laid a white floral wreath at the Pentagon, above a concrete slab that said "Sept. 11, 2001 — 9:37 a.m."

In remarks at the Pentagon, Obama recalled a day "when grief crashed over us like an awful wave."

But many felt that last year's 10th anniversary was an emotional turning point for public mourning of the attacks. For the first time, elected officials weren't speaking at the Ground Zero ceremony, which often allowed them a solemn turn in the spotlight, but raised questions about the public and private Sept. 11.

"I feel much more relaxed" this year, said Jane Pollicino, who came to Ground Zero Tuesday morning to remember her husband, who was killed at the trade centre. "After the ninth anniversary, that next day, you started building up to the 10th year. This feels a lot different, in that regard. It's another anniversary that we can commemorate in a calmer way, without that 10-year pressure."

Fewer gather for 11th anniversary

Thousands had attended the ceremony in New York in previous years, including last year's milestone 10th anniversary. Fewer than 500 family members had gathered by Tuesday morning.

U.S. President Barack Obama and Michelle Obama pause during a moment of silence to mark the 11th anniversary of the Sept, 11 attacks, on the South Lawn of the White House in Washington. (Carolyn Kaster/Associated Press)

As bagpipes played at the year-old Sept. 11 memorial in New York, families bowed their heads in silence at 8:46 a.m., the moment that the first hijacked jetliner crashed into the trade centre's North Tower, and again to mark the crashes into the South Tower, the Pentagon and a Pennsylvania field.

More than four million people have visited the memorial in the past year. On Tuesday, much of downtown Manhattan bustled like a regular weekday, except for clusters of police and emergency vehicles on the borders of the site.

Joe Torres, who put in 16-hour days on the site in the days after the attacks, cleaning up tons of debris, said another year has changed nothing for him.

"The 11th year, for me, it's the same as if it happened yesterday. It could be 50 years from now, and to me, it'll be just as important as year one, or year five or year 10," Torres said.

Costly wars followed Sept. 11

The attacks were followed by wars in Afghanistan and Iraq, where the U.S. military death toll years ago surpassed the Sept. 11 victim count. At least 1,987 U.S. troops have died in Afghanistan and 4,475 in Iraq, according to the Pentagon.

One World Trade Center, now up to 105 floors, rises above the National September 11 Memorial and Museum in New York. (Mark Lennihan/Associated Press)

Allied military forces marked the anniversary at a short ceremony at NATO's headquarters in Kabul, Afghanistan, with a tribute to more than 3,000 foreign troops killed in the decade-long war.

"Eleven years on from that day, there should be no doubt that our dedication to this commitment, that commitment that was seared into our souls that day so long ago, remains strong and unshaken," said marine Gen. John Allen, the top commander of U.S. and coalition troops.

The Obamas visited the graves of service members killed in Afghanistan and Iraq at Arlington National Cemetery Tuesday morning. They quietly walked between rows of graves at Section 60, which contains the remains of the most recent war dead.

Pausing at several graves, Obama placed presidential "challenge" coins at the base of the headstones. The first headstone listed the names of 10 victims of an Oct. 26, 2009, helicopter crash in Afghanistan.

The Obamas planned later to visit wounded soldiers at Walter Reed Army Medical Center.

Other ceremonies were held across the country, but some cities scaled back.

The National Sept. 11 Memorial and Museum announced this summer that this year's ceremony would include the words of family members, hoping to remember the dead and honour families "in a way free of politics" in an election year, memorial president Joe Daniels said.

Yasmin Leon, whose sister was killed at the trade centre, said there was a sense of closure this year now that the Sept. 11 memorial — twin reflecting pools surrounded by victims' names — was open to the public.

"This year, we're just here to reflect," she said.

The Sept. 11 museum was initially to open this year, but is on hold for at least another year after a months-long dispute over financing between the foundation and the government agency that owns the site. Late Monday, New York Mayor Bloomberg and New York Gov. Andrew Cuomo announced an agreement that paves the way for finishing the $700 million-plus project "as soon as practicable."

Harper notes sacrifice

Canadian Prime Minister Stephen Harper issued a statement on the anniversary of the attacks, calling them “a day that shocked the world.”

Harper noted that among the nearly 3,000 killed in the attacks were 24 Canadians, who are also remembered and mourned.

“We also honour the incredible acts of courage and sacrifice of those who responded to the calls for help of those in need. From the police officers, firefighters and emergency personnel who risked or gave their lives to help others, to the small community in Gander, Newfoundland, which welcomed thousands of diverted air passengers into their homes.”