US President Barack Obama and his wife Michelle stood on the White House lawn on Friday to observe a moment of silence to mark the 14th anniversary of the 9/11 attacks.

The silence was observed at 8:46 am - the time when the first jet commandeered by al-Qaida operatives smashed into the north tower of the World Trade Center in New York, the site for a ceremony of remembrance.

Later, Obama was scheduled to observe the anniversary with a visit to Fort Meade, Maryland, in recognition of the military's work to protect the country.

The attacks killed nearly 3,000 people in New York, at the Pentagon and aboard a hijacked airliner that went down in Pennsylvania.

At the White House, a few hundred people - political staff members including National Security Adviser Susan Rice, but also the White House chefs, gardeners and other residence staff - joined the Obamas for the ceremony.

In New York, police and victims' relatives began marking the anniversary in a subdued gathering at ground zero, now the site of the National 9/11 Memorial and Museum, with a moment of silence and somber reading of names.

Hundreds of victims' relatives - fewer than thronged the ceremonies in the early years - attended.

They carried photos emblazoned with the names of their lost loved ones.

Obama leads moment of silence on 9/11 anniversary

"We come every year. The crowds get smaller, but we want to be here. As long as I'm breathing, I'll be here," said Tom Acquaviva, 81, who lost his son Paul, a systems analyst who died in the trade center's north tower.

Nereida Valle, who lost her daughter, Nereida De Jesus, said: "It's the same as if it was yesterday. I feel her every day."

New York Mayor Bill de Blasio said on Twitter: "Emotions and memories of 9/11 are with us so vividly today and always. We will never forget our loved ones."

The Flight 93 National Memorial near Shanksville, Pennsylvania, was marking the completion of its visitors' center, which opened to the public on Thursday.

At the Pentagon, Defense Secretary Ash Carter and other officials were joining in remembrances for victims' relatives and Pentagon employees.

After years of private commemorations at ground zero, the anniversary has also become an occasion for public reflection at the site of the attacks.

An estimated 20,000 people flocked to the memorial plaza on the evening of Sept 11 last year, the first year the public was able to visit on the anniversary.