In this photo released by US Department of Defense (DOD), US President Barack Obama (L), Defense Secretary Chuck Hagel (C) and Chairman of the Joint Chiefs of Staff Martin Dempsey attend a September 11th remembrance ceremony at the Pentagon in Washington, capital of the United States, Sept 11, 2013. [Photo/Xinhua/DOD Photo]

NEW YORK/WASHINGTON - Amid moments of silence and a reading of victims' names, Americans paused in solemn ceremonies Wednesday to cherish the memory of the nearly 3,000 innocent people who died when hijacked jetliners crashed into the World Trade Center, the Pentagon and a Pennsylvania field 12 years ago.

More than a thousand people gathered at the National September 11 Memorial plaza in lower Manhattan, New York City, for the annual reading of victims' names from both 1993 and 2001 terror attacks on the World Trade Center, against the background of playing bagpipes and tolling bells.

The solemn ceremony was held around two reflecting pools in the footprints of the twin towers on the former World Trade Center complex. Participants first observed a moment of silence at 8:46 am EDT (1246 GMT), the time when American Airlines Flight 11 crashed into the North Tower and there was a second pause at 9:03 am when United Airlines Flight 175 struck the South Tower.

In Washington, President Barack Obama, joined by Vice-President Joe Biden and other members of the White House, also marked the moment on the South Lawn of the White House Wednesday morning.

Obama, first Lady Michelle Obama, Biden, and Jill Biden stood in silence as a bell tolled and than slowly walked back inside the White House.

Later in the morning, the president attended another observance ceremony at the Pentagon Memorial with victims' families, survivors and military officials. He laid a wreath at the base of the Pentagon Memorial and paused for a moment of silence at 9:37 am to mark the time that American Airlines Flight 77 struck the Pentagon.

"We pray for the memory of all those taken from us -- nearly 3,000 innocent souls," said Obama, stressing that 12 years on, the nation's hearts still ache for the futures snatched away in the terror attack.    

Members of US Congress also held a ceremony on the Capitol Hill steps to mark the anniversary. Similar memorial service is also held in Shanksville, Pennsylvania, where the United Flight 93 hit the ground at 10:03 am.

"When you think of them, think what happened, see the place we are on today still at war. I think we all should slow down and think of people who died," Partik Lalander, a Swedish tourist in New York City, told Xinhua.

Nineteen hijackers were killed in the suicide attacks, for which Osama bin Laden and al Qaeda claimed credit, leading to the US war in Afghanistan and in Iraq.

Making no direct mention of the Syrian crisis, President Obama appealed for the strength to face the threats that endure and vowed to "stand vigilant and defend our nation".

He also paid tribute to the four Americans who were killed one year ago in a terrorist attack on the US diplomatic mission in Benghazi, Libya.

The observance came amid extensive diplomatic efforts by the Obama administration in response to the alleged use chemical weapons in Syria. A potential punitive strike against Syria has met with strong opposition among Americans weary of wars in Iraq and Afghanistan.  

Worries over Syria waned as US President Barack Obama said Tuesday night that the United States will make joint efforts with Russia and other partners to work on a United Nations resolution making Syrian President Bashar al-Assad's government give up chemical weapons. He also asked Congress to postpone a vote on the military strike against Syria, and let more time for diplomacy to work.

There are glimmers of hope as life goes on. Twelve years later, two skyscrapers have been almost finished on either side of the National September 11 Memorial plaza, including One World Trade Center, the tallest building in the Western Hemisphere with a symbolic height of 1,776 feet (541 meters), a number chosen to mark the year of the US Declaration of Independence.