Obama marks 11th anniversary of 9/11 attacks

President Barack Obama marked the anniversary of the 9/11 attacks, stating that the United States had emerged "even stronger" during one of three main ceremonies commemorating the 11th anniversary of the attacks in which nearly 3,000 people lost their lives after airliners were hijacked by Islamist militants.

Two of the passenger planes brought down the Twin Towers of New York City's World Trade Center while another hit the Pentagon outside Washington. A fourth crashed in a field in rural Pennsylvania after passengers fought back against the hijackers.

Speaking at the Pentagon where 184 people were killed, Obama told victims' families that the entire nation shared their loss.

"Eleven times we have paused in remembrance and reflection, in unity and in purpose…This is never an easy day, but it is especially difficult for all of you, the families of nearly 3,000 innocents who lost their lives,” said Obama.

"But no matter how many years pass, no matter how many times we come together on this hallowed ground, know this: That you will never be alone, your loved ones will never be forgotten. They will endure in the hearts of our nation because through their sacrifice they helped us make the America we are today, an America that has emerged even stronger."

Speaking under a cloud free blue sky, reminiscent of the morning of September 11, 2001, Obama said America's fight was not with Islam but with al Qaeda, the group responsible for the attacks.

It is a stance he has taken on numerous occasions since taking office in a bid to mend ties with the Muslim world.

"I've always said our fight is with al Qaeda and its affiliates, not with Islam or any other religion," he said. "This country was built as a beacon of freedom and tolerance."

Before the ceremony, Obama and First Lady Michelle Obama observed a moment of silence on the South Lawn of the White House.

They later paid a visit to Arlington National Cemetery to offer their respects at the graves of military service members killed in Iraq and Afghanistan.

Meanwhile in Shanksville, Pennsylvania, Vice President Joe Biden addressed the families of the 40 passengers who died aboard United Airlines Flight 93 after they confronted their hijackers, resulting in the plane crashing in a remote field.

U.S. authorities say the hijackers intended to crash the plane into the U.S. Capitol in Washington.

Americans have "not forgotten the heroism of your husbands, wives, sons, daughters, mothers, fathers. And that what they did for this country is still etched in the minds of not only you, but millions of Americans, forever," Biden told a crowd gathered at the crash site, now a national memorial.

Obama's rival in the race to the White House, Republican presidential nominee Mitt Romney, said the anniversary marked a day "when evil descended upon our country."

"On this most somber day, those who would attack us should know that we are united as one in our determination to destroy them and to stand tall for peace and freedom at home and across the world," Romney said in a statement.

Both Obama and Romney suspended negative advertising for the day.