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

US President Barack Obama led the United States 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, in Afghanistan, 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.

"We honor 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 Center 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 Defense Secretary Ashton Carter placed a large wreath of white flowers.

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

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.