In silence, in prayers

Americans commemorated the 15th anniversary of the Sept 11, 2001 attacks yesterday with a moment of silence, 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.

The remnant of the Twin Towers after the terror attack. Photo: AFP

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 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.

Smoke billows from the World Trade Center on September 11, 2001 in New York after hijacked commercial planes crashed into them. Photo: AFP

"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.

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.