The twelfth anniversary of the September 11, 2001, al-Qaeda attacks has been marked throughout the US with the reading of victims' names and moments of silence.

At Wednesday's morning ceremony at Ground Zero in New York City, relatives recited the names of the nearly 3,000 people who died when hijacked jets crashed into the World Trade Centre, the Pentagon and near Shanksville, Pennsylvania, as well as the 1993 World Trade Centre bombing victims' names.

"No matter how many years pass, this time comes around each year - and it's always the same," said Karen Hinson of Seaford, New York, who lost her 34-year-old brother, Michael Wittenstein, a Cantor Fitzgerald employee.

"My brother was never found, so this is where he is for us," she said as she arrived for the ceremony with her family early on Wednesday.

In Washington DC, President Barack Obama joined First Lady Michelle Obama, Vice President Joe Biden, Jill Biden and members of the White House staff in a moment of silence on the White House South Lawn.

Afterwards, in a memorial service at the Pentagon, Obama called on Americans to pray for those whose lives had been lost.

"Let us have the strength to face the threats that endure, different though they may be from 12 years ago, so that as long as there are those who would strike our citizens, we will stand vigilant and defend our nation," he said.

The day was marked with spells of silence that symbolised the key moments of the attacks.

In the first of six moments of silence, at 8.46am, the moment that the first hijacked passenger jet crashed into the north tower of the World Trade Centre, many Americans, including Obama, fell silent.

For a slideshow of images from today's anniversary, visit our Al Jazeera America gallery.

Source: Al Jazeera and agencies