President Barack Obama lauded American unity Tuesday as the country marked a somber but low-key anniversary of the September 11, 2001 attacks under crisp blue skies poignantly reminiscent of 11 years ago.

"The true legacy of 9/11 will not be one of fear or hate or division," Obama said at the Pentagon near Washington. "It will be a safer world, a stronger nation, and a people more united than ever before."

Highlighting what he said were the "crippling" blows dealt against Al-Qaeda and the killing last year of Osama bin Laden, Obama said the United States is "even stronger."

As every year, relatives of the nearly 3,000 people killed when al-Qaeda hijackers slammed airliners into New York's World Trade Center gathered at Ground Zero to read out the names of the dead.

The flawless blue sky was identical to the one 11 years ago when millions of people watched from the streets and live on television as the planes flew straight into the upper floors of the Twin Towers, causing them to collapse.

However, emotions are distinctly cooler as America finally tries to draw a line under an event that sparked the decade of Washington's controversial and expensive war on terror.

No politicians joined in the reading at Ground Zero and security was less intense, in contrast to the 10th anniversary last year when Obama headed a long list of VIPs at the ceremony.

June Pollicino, who lost her husband on 9/11, told AFP: "I feel much more relaxed. After the ninth anniversary, those next days it started building up to the 10th anniversary. This year it's different in that regard. It's another anniversary we can celebrate in a discreet way."

Although most New York area newspapers featured front page stories or other mentions about the anniversary on Tuesday, The New York Times and the tabloid Post were conspicuous in deciding to keep coverage inside.

In total, 2,983 names were read out at Ground Zero, including the 9/11 victims and those killed in the precursor to those attacks, the 1993 car bombing of the World Trade Center.

The reading paused for silence at the exact time each of the four planes turned into fireballs -- two smashing into the Twin Towers, one into the Pentagon and one into a Pennsylvania field.

Another two moments of silence were observed at the times the two main towers of the World Trade Center collapsed, accounting for the vast majority of 9/11's dead.

Obama, who earlier stood for a moment's silence on the White House South Lawn, had no planned political events planned Tuesday and his reelection campaign planned to halt television advertising for the day, a campaign official said.

However, there was no formal pause similar to that in the presidential campaign of four years ago, when both Obama and his then Republican rival John McCain joined to lay a wreath at Ground Zero in New York.

Former president Bill Clinton was campaigning for his fellow Democrat Obama and speaking out against Republican Mitt Romney in Miami. Romney issued a statement thanking US troops and saying "those who would attack us should know that we are united."

Vice President Joe Biden, meanwhile, was traveling to Shanksville, Pennsylvania, where United flight 93 crashed after passengers attacked the hijackers and prevented them from hitting another presumed high-profile target, such as the US Capitol building.

The passage of time and more pressing worries about the moribund US economy have distracted public attention from the tragedy of 9/11, particularly compared to the huge media coverage of the 10th anniversary last year.

Helping to heal the wounds are the new memorial at Ground Zero and the near completion of main skyscraper at the World Trade Center, now officially the tallest building in New York.

The memorial's long delayed museum now also appears set to be opened after Mayor Michael Bloomberg and New York Governor Andrew Cuomo reached an accord late Monday over funding.

Underlining US successes in targeting al-Qaeda leaders, bin Laden's successor Ayman al-Zawahiri released a video on the eve of this year's anniversary in which he confirmed that his deputy, Abu Yayha al-Libi, had been killed in a drone strike in Pakistan in June.

Libi was considered Al-Qaeda's global propaganda mastermind and his death dealt the biggest blow to the group since the killing of bin Laden in May 2011.

However, the Taliban scorned the idea that they are defeated, saying in Afghanistan that they had nothing to do with 9/11 and that the United States faces "utter defeat in Afghanistan militarily, politically, economically and in all other facets."

Most foreign troops are scheduled to withdraw by the end of 2014, handing over responsibility for combat to Western-backed Afghan government forces.