US President Barack Obama speaks during a ceremony marking the 13th anniversary of the 9/11 attacks at the Pentagon in Washington on September 11, 2014.[Photo/Agencies]

NEW YORK - Led by President Barack Obama, Americans commemorated the 13th anniversary of the Sept. 11, 2001, attacks on Thursday by observing moments of silence for the thousands killed that day at New York City's World Trade Center, the Pentagon and a Pennsylvania field.

In what has become an annual ritual, relatives began slowly reciting the nearly 3,000 names of the victims at a ceremony in lower Manhattan, from Gordon Aamoth Jr. to Igor Zukelman.

Readers would occasionally pause as a silver bell was rung to mark the exact times when each of the four planes hijacked by al Qaeda militants crashed at the three sites and when each of the World Trade Center's twin towers collapsed. With each bell, a moment of silence was observed.

Obama spoke at the Pentagon during a private ceremony for relatives of the 184 people killed in the attack on the US Department of Defense headquarters in Arlington, Virginia, several miles from the White House.

He laid a wreath of white lilies and chrysanthemums, and kept his hand on his heart as "Taps" played.

"Thirteen years after small, hateful minds conspired to break us, America stands tall and America stands proud," Obama said.

In New York, the voice of Tom Monahan, a 54-year-old man with salt-and-pepper hair and broad shoulders, cracked when he talked about the brother and cousin he lost in the attack.

"Everything is fine until you get here," he said before waving his hands as if to signal he could not talk anymore. He emerged from the security checkpoints an hour later and showed a reporter a message he had sent on his cell phone to his sister. "9-12 couldn't come soon enough," it said.

Beyond the checkpoints, an invitation-only crowd stood beneath an overcast sky in the memorial plaza at the heart of the new World Trade Center, which is nearing completion in lower Manhattan. Some of those in attendance were dressed in military uniform, others wore T-shirts and sneakers.

Many people held up posters with smiling photographs of their dead relatives. Red roses and American flags poked up from the bronze plates bearing victims' names that ring the two waterfalls that now trace the footprints of the fallen towers.

New York Governor Andrew Cuomo, New York City Mayor Bill de Blasio and two former mayors, Michael Bloomberg and Rudy Giuliani, were among the mourners.

The high fences blocking off public access to most of the World Trade Center site finally came down in May.

While lower Manhattan may look different this year, the threat to the United States represented by the Sept. 11 attacks remains. Washington and its allies see Islamic State, a group that began as an offshoot of al Qaeda, as an increasing danger.

On Wednesday, Obama said he had ordered an aerial bombing campaign targeting the group, which has seized large parts of Iraq and Syria and released videos of beheadings of two American journalists.

"It definitely drives home the fact that there are certain things that haven't changed since September 11th," Brendan Chellis, who was working on the 30th floor of one of the twin towers at the time of the attack, said outside the New York ceremony.

The only ceremony open to the general public was at the Flight 93 National Memorial in Shanksville, Pennsylvania, where one of the four hijacked airliners crashed after a struggle between passengers and the hijackers.

George Meyers, a 43-year-old paralegal, was living in Shanksville 13 years ago.

"I felt the ground shake the day it happened," he said during a visit to the memorial, set amid bucolic rolling fields. "It's hard to come outside and see grieving families but it's nice to see them smile at the memorial that's been built."