Memorial ceremonies marking the anniversary of the September 11 attacks began yesterday under clear blue skies that recalled the crisp morning 11 years ago when nearly 3000 people were killed by aircraft hijacked by Islamist militants.

Two of the planes brought down the Twin Towers of New York City's World Trade Centre, another damaged the Pentagon outside Washington and a fourth crashed in a field in rural Pennsylvania when passengers aboard that flight revolted against the hijackers.

At Ground Zero, where the two towers once stood, more than 1000 relatives of those killed and others gathered for the annual reading of the list of 2983 people who died at the three sites. The list excludes the 19 hijackers, who also died.

The reading began at 8.39am with pauses for moments of silence at 8.46am, 9.03am, 9.37am and 10.03am, the time of impact for the four planes, and again at 9.59am and 10.28am, the times the north and then the south tower fell.

As the moment of the reading approached, family members, uniformed police and firefighters milled about the vast, twin reflecting pools that mark the footprints of the two towers, their edges etched with the names of the victims. Many carried or wore pictures of their loved ones.

Alyson Low, 41, of Fayetteville, Arkansas, carried a picture of her sister, Sara Elizabeth Low, who was a flight attendant on American Airlines Flight 11, the first plane to crash, striking the trade centre's north tower.

"I'm tired," Low said, tearfully.

"I am just so tired."

The reading of names began with Patricia Abbott, wife of Alan Jay Richman, who died at the trade centre.

It took more than three hours by 198 people to read the list alphabetically. In previous years, politicians, including US presidents, governors and mayors, have participated in the reading of the names, or have read from the Bible or recited passages from literature.

This year only the families of people who were killed at the World Trade Centre would appear on the podium to read their names.

Under event rules set down in July by the National September 11 Memorial and Museum, chaired by New York Mayor Michael Bloomberg, politicians were not allowed to participate in the reading of names.

President Barack Obama observed a moment of silence for the September 11 victims on the South Lawn of the White House. Flanked by a flag-bearing military honour guard, the president and First Lady Michelle Obama stood solemnly with heads bowed.