NEW YORK (AFP) - America marked the 20th anniversary of 9/11 on Saturday (Sept 11), with solemn ceremonies given added poignancy by the recent chaotic withdrawal of troops from Afghanistan and the return to power of the Taliban.

At the 9/11 memorial in New York, a moment's silence was held at 8.46 am, the time the first hijacked plane crashed into the World Trade Centre on Sept 11, 2001.

Relatives, many wiping away tears, then began reading the names of those killed as the four-hour-long ceremony, attended by US President Joe Biden and former commander-in-chief Barack Obama, got under way under tight security in New York.

Heart-wrenching commemorations will also unfold at the two other sites - the Pentagon in Washington DC and Shanksville in Pennsylvania - targeted by the Al-Qaeda.

The memorials come with US troops finally gone from Afghanistan, but national discord - and for President Biden, political peril - is overshadowing any sense of closure.

In a video posted on the eve of the anniversary, Mr Biden urged Americans to show unity, "our greatest strength".

"To me, that's the central lesson of Sept 11. It's that at our most vulnerable, in the push and pull of all that makes us human, in the battle for the soul of America, unity is our greatest strength," Mr Biden said in a six-minute message from the White House.

At New York's Ground Zero, where two pools of water now stand where the Twin Towers used to, relatives read out the names of the nearly 3,000 people killed, in a four-hour-long service that started at 8.30am local time.

Six moments of silence will be observed, corresponding with the times the two World Trade Centre towers were struck, and fell, and the moments the Pentagon was attacked and Flight 93 crashed.

Ms Monica Iken-Murphy, who lost her 37-year-old husband Michael Iken in the World Trade Center, said this will be a "heightened" anniversary for many Americans.

But for her, as for many other survivors, the pain has never wavered. "I feel like it just happened," she said.

A whole generation has grown up since the morning of Sept 11, 2001.

In the interim, Al-Qaeda founder Osama bin Laden has been hunted down and killed. A towering new sky scraper has risen over Manhattan, replacing the Twin Towers. And less than two weeks ago, the last US soldiers flew from Kabul airport, ending the so-called "forever war".

But the Taliban that once sheltered bin Laden is back ruling Afghanistan, the mighty US military humiliated. In Guantanamo Bay, accused 9/11 mastermind Khalid Sheikh Mohammed and four other men continue to await trial, nine years after charges were filed.

Even the full story of how the attack came to happen remains secret. Only last week did Mr Biden order the release of classified documents from the Federal Bureau of Investigation investigation over the next six months.

At Ground Zero in 2001, some 2,753 people from all over the world were killed in the initial explosions, jumping to their deaths, or simply vanishing in the inferno of the collapsing towers.

At the Pentagon, an airliner tore a fiery hole in the side of the superpower's military nerve centre, killing 184 people in the plane and on the ground.

And in Shanksville, Pennsylvania, the third wave of hijackers crashed into a field after passengers fought back, sending United 93 down before reaching its intended target - likely the US Capitol building in Washington.

Mr Biden and First Lady Jill Biden will stop at each of these places on Saturday to "honour and memorialise the lives lost", the White House said.

In his video address on Friday night, Mr Biden urged Americans to come together as they reflect on the tragedy.

"Unity doesn't mean we have to believe the same thing, but we must have a fundamental respect and faith in each other and in this nation," he said.

The President had planned for this to be a pivotal day in his nearly eight-month-old presidency.

However, instead of presiding over a moment of unity, Mr Biden will traverse a country angry about the messy Kabul evacuation, which included 13 US soldiers killed by a suicide bomber, and stung by the broader realisation of failure and defeat.

For the relatives of victims, the anniversary, as always, is about keeping the memory of their loved ones alive.

"It's like Pearl Harbour," said Mr Frank Siller, whose firefighter brother Stephen died at the World Trade Center. "People who weren't alive don't have the same feeling about it as those who were alive. But America has never forgotten about Pearl Harbour and America will never forget about 9/11."