WASHINGTON -- Twelve years after experiencing the worst attack in the history of the United States, US President Barack Obama held a moment of silence on the South Lawn of the White House with his wife and vice president on the minute of its anniversary, 8:46, marking a sober new tradition required of American presidents.

Flanked by honor guard, the president held his hand over his heart. The South Portico of the White House held its flag at half staff.

Obama then traveled a short distance to the Pentagon, where he again placed his hand over his heart after laying a wreath at the Zero Age Line, the memorial to those who died when a plane hit the military complex.

In brief remarks, the president said that force is sometimes necessary to protect the American people and their ideals, but that it cannot forge the world we wish to ultimately live in.

"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," Obama said.

"Let us have the wisdom to know that while force is at times necessary, force alone cannot build the world we seek."

Chairman of the joint chiefs of staff Martin Dempsey said that colorful quilts now decorate the restored halls of the Pentagon section devastated by the plane's impact. Defense Secretary Chuck Hagel called the president "a leader of wisdom and compassion."

The two men have, with Secretary of State John Kerry, spent the last week in hearings with members of Congress making the case for new strikes in Syria.

Citing scripture in his speech, the president said the US had undergone a "restoration."

"In your resilience," he said, "you taught us all there's no trouble we can't endure and no calamity that we can't overcome."

Meanwhile, bagpipes, bells and a reading of the names of the nearly 3,000 people killed when hijacked jetliners crashed into the World Trade Center, the Pentagon and a Pennsylvania field marked the 12th anniversary of the Sept. 11 attacks.

While crowds gathered in Washington to hear Obama, more than a thousand people gathered Wednesday on a hot and hazy morning at the National September 11 Memorial plaza in Manhattan, for the annual reading of victims' names from both the 1993 and 2001 attacks on the World Trade Center.

Bagpipes and a youth choir ushered in the start of the solemn proceedings, held around two reflecting pools that stand in the footprint of the fallen twin towers.

"To my nephew Michael Joseph Mullin, we miss you and think of you every single day," said one of the 250 people chosen to read names. "You're gone but not forgotten," another woman said of her lost cousin.

In keeping with a tradition that began last year, no public officials spoke at the New York ceremony, though former New York Mayor Rudolph Giuliani, his successor Michael Bloomberg, Police Commissioner Raymond Kelly, and other city and state leaders were in attendance.

The morning after a speech in which he urged Americans to support his proposal to use military force against Syria, in retribution for President Bashar Assad's's poison gas attack on his own people, Obama also reflected on the limits of force.

"Let us have the wisdom to know that, while force is at times necessary, force alone cannot build the world we seek," Obama said.

Moments of silence

Americans observed moments of silence at 8:46 a.m. EDT (1246 GMT), the time American Airlines Flight 11 crashed into the North Tower and with a second pause at 9:03 a.m. (1303 GMT) when United Airlines Flight 175 struck the South Tower.

Further moments of silence were observed at 9:37 a.m. (1337 GMT), when American Airlines Flight 77 hit the Pentagon; at 9:59 a.m. (1359 GMT) when the South Tower fell; at 10:03 a.m. (1403 GMT) when United Flight 93 hit the ground near Shanksville, Pennsylvania; and at 10:28 a.m. (1428 GMT), when the North Tower collapsed.

Nineteen hijackers died in the attacks, later claimed by Osama bin Laden and al-Qaida, which led directly to the US war in Afghanistan and indirectly to the invasion of Iraq.

Two skyscrapers are nearly completed on either side of the plaza, including One World Trade Center, the tallest building in the Western Hemisphere at 1,776 feet (541 meters), a symbolic number chosen to allude to the year of the Declaration of Independence.

At the Flight 93 National Memorial in Pennsylvania, the National Park Service's memorial service Wednesday included a reading of the names of the Flight 93 passengers and crew, a ringing of bells, a wreath-laying and brief remarks.

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