Victims' relatives have been marking the 14th anniversary of the September 11 attacks in a subdued gathering at Ground Zero in New York, with a moment of silence and sombre reading of names.

Hundreds of relatives - fewer than thronged the ceremonies in their early years - gathered, carrying photos emblazoned with the names of their loved ones as they remembered the day when hijacked planes hit the World Trade Centre's twin towers, the Pentagon and a field near Shanksville, Pennsylvania.

"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 World Trade Centre's north tower.

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

President Barack Obama and first lady Michelle Obama stepped out of the White House at 8.46am - when the first plane hit the north tower - to observe a moment of silence.

The Flight 93 National Memorial near Shanksville in western Pennsylvania was marking the completion of its visitor centre, which opened to the public on Thursday. At the Pentagon, defence secretary Ash Carter and other officials joined 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 on the site of the terror attacks.

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

"When we did open it up, it was just like life coming in," National September 11 Memorial and Museum president Joe Daniels said this week. While the memorial will be reserved for victims' relatives and other invitees during the morning ceremony, afterwards "the general public that wants to come and pay their respects on this most sacred ground should be let in as soon as possible".