Americans commemorated the 15th anniversary of the September 11, 2001 attacks on Sunday with the recital of the names of the dead, tolling church bells and a tribute in lights at the site where New York City's massive twin towers collapsed.

As classical music drifted across the 9/11 Memorial plaza in lower Manhattan, family members and first responders slowly read the names and delivered personal memories of the almost 3,000 victims killed in the worst attack on US soil since the 1941 bombing of Pearl Harbour.

Tom Acquarviva lost his 29-year-old son Paul, who worked at financial services firm Canter Fitzgerald on the 101st to 105th floors of the North Tower, just above where the first plane struck. Acquarviva was one of 658 Cantor Fitzgerald employees killed in the attack.

"We miss him terribly. Terribly, terribly, terribly. Not a day goes by that we don't remember him," Acquarviva told Reuters. But he said he felt a sense of hope: "There are more people here today than there ever have been."

The ceremony paused for six moments of silence: four to mark the exact times four hijacked planes were crashed into the World Trade Center, the Pentagon near Washington D.C., and a Pennsylvania field. The last two record when the North and South towers of the Trade Center crumpled.

It was held by two reflecting pools with waterfalls that now stand in the towers' former footprints, and watched over by an honour guard of police and firefighters.

More than 340 firefighters and 60 police were killed on the that sunny Tuesday morning in 2001. Many of the first responders died while running up stairs in the hope of reaching victims trapped on the towers' higher floors.

At the Pentagon, a trumpet played as US President Barack Obama took part in a wreath-laying ceremony.

"Fifteen years may seem like a long time. But for the families who lost a piece of their heart that day, I imagine it can seem like just yesterday," Obama said.

No public officials spoke at the New York ceremony, in keeping with a tradition that began in 2012. But many dignitaries attended, including Republican presidential hopeful Donald Trump and his Democratic rival Hillary Clinton.

"We'll never forget the horror of Sept. 11, 2001," Clinton said in a brief statement. "Let's honor the lives and tremendous spirit of the victims and responders."

Trump said in a statement that it was a day of sadness and remembrance, but also of resolve.

"Our solemn duty on behalf of all those who perished ... is to work together as one nation to keep all of our people safe from an enemy that seeks nothing less than to destroy our way of life," Trump said.

TRIBUTE IN LIGHTS

Houses of worship throughout the city tolled their bells at 8:46 a.m. EDT (1246 GMT), the time American Airlines Flight 11 slammed into the North Tower.

A second pause came at 9:03 a.m. (1303 GMT), when United Airlines Flight 175 struck the South Tower. American Airlines Flight 77 hit the Pentagon at 9:37 a.m. (1337 GMT), then the South Tower collapsed at 9:59 a.m. (1359 GMT).

At 10:03 a.m. (1403 GMT) United Flight 93 crashed near Shanksville, Pennsylvania, and the final moment of silence was observed at 10:28 a.m. (1428 GMT) when the North Tower fell.

As evening falls across New York City on Sunday, spotlights will project two giant beams of light into the night sky to represent the fallen twin towers, fading away at dawn.

In the twin towers' place now rises the 104-story 1 World Trade Center. Also known as the Freedom Tower, it is the tallest skyscraper in the Western Hemisphere, at 1,776 ft. Fifteen years after the attack, the US government marked its return to the site on Friday, moving its New York City offices there.

Nineteen hijackers died in the attack, later claimed by Osama bin Laden and al Qaeda, which led directly to the US war in Afghanistan and indirectly to the invasion of Iraq.

In Kabul, the top American commander in Afghanistan, General John Nicholson, paid tribute to members of the Nato-led coalition and Afghan security forces who had been killed since the Taliban regime fell.

But in an address which touched on his own experience as an officer in Afghanistan, stretching back a decade, he also underlined how far from peace the country remains.

"As we know, sadly, the number of terrorist groups has only grown since 9/11," he said. "Of the 98 groups now designated globally, 20 are in this region, the Afpak region."-Reuters