WASHINGTON: President Barack Obama led the United States (US) Friday in remembrance of 9/11, urging Americans to remain vigilant of "terrorist' threats on the 14th anniversary of the attacks.

Although US forces "have made enormous strides in degrading the core Al-Qaeda," the terror group responsible for the deadly strikes on US soil, “we are well aware of the fact that those threats still exist out there," Obama said in a speech broadcast live to US service members worldwide.

"Both in Iraq and in Syria, Afghanistan and in North Africa, what we're very clear about is we have significant threats coming from terrorist organizations and the terrorist ideology," Obama warned from Fort Meade, Maryland.

Earlier in the day, at 8:46 am, a bell chimed three times on the South Lawn of the White House to mark the moment when Flight 11, piloted by Al-Qaeda operatives, careened into the North Tower of the World Trade Centre in New York.

Blue skies and the hum of jet planes landing and taking off at nearby National Airport evoked that day of tragedy.

Obama and his wife Michelle stood solemnly beneath a US flag at half-staff, bowed their heads and marked a moment of silence.

The Tribute in Light is seen on the 14th anniversary of the 9/11 attacks on the World Trade Center at the National September 11 Memorial and Museum in Lower Manhattan in New York September 11, 2015. — REUTERS

The first couple were flanked by White House chefs, gardeners and housekeepers, as well as national security staff.

Evidence of 9/11's impact was everywhere — from Obama's stars and stripes lapel pin, now ubiquitous among US politicians, to the presence of Lisa Monaco, his Homeland Security Advisor — a post that did not exist before the attacks.

Nearly 3,000 people died on September 11, 2001 at Ground Zero in New York, at the Pentagon and aboard a hijacked airliner that went down in rural Pennsylvania.

"We honour those we lost. We salute all who serve to keep us safe. We stand as strong as ever," Obama later said in a post to social media.

Almost a decade and a half later, Osama bin Laden is dead and the US presence in Afghanistan and Iraq has ebbed, but Americans' sense of loss and shock has receded little.

In New York, police and relatives of those killed in the World Trade Centre read the names of the victims at Ground Zero, now the site of the National 9/11 Memorial and Museum.

At the Pentagon, dozens of family members watched as Defence Secretary Ashton Carter placed a large wreath of white flowers.

"They did not and could not take from us what defines us," Carter said.

'Forever war'

As commemorations across the eastern US replicated the timeline and solemn geography of September 11, 2001, there was also a reminder that the threat posed by Islamist terror groups remains both clear and present.

"The war that began 14 years ago still rages around the world today," said Senator John McCain.

"With the forces of radical Islam once again ascendant in the Middle East and North Africa, we must aspire to recapture the spirit of unity that marked our public life in the wake of the 9/11 attacks," he said.

The US must, he said, "devote ourselves with firm resolve to the lasting defeat of the enemies that attacked us that day, and who seek to attack us still today."

Counter-terror analysts were closely watching for threats from Al-Qaeda or the self-styled Islamic State group.

In 2012, a September 11 attack on the US consulate in Benghazi, Libya killed four Americans, including Ambassador Chris Stevens.

Federal Bureau of Investigation chief James Comey said there were "not any specific or credible threats" this year, but that authorities were on alert.