The United States has commemorated the 15th anniversary of the 9/11 attacks 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 Harbor.

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. He 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," Mr 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 DC, 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 honor guard of police and firefighters.

More than 340 firefighters and 60 police were killed on that 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," Mr 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 September 11, 2001," Mrs Clinton said in a brief statement. "Let's honor the lives and tremendous spirit of the victims and responders."

Mrs Clinton left the ceremony feeling unwell and her campaign team subsequently announced she was diagnosed with pneumonia on Friday.

Mr 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," Mr Trump said.

As evening falls across New York City, 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-storey 1 World Trade Center.

Also known as the Freedom Tower, it is the tallest skyscraper in the Western Hemisphere, at 541m.

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.