WASHINGTON (AFP) - US President Barack Obama on Sunday urged Americans on the 15th anniversary of the 9/11 attacks to embrace the nation's diversity and not to allow "terrorists" to divide the country.

Extremist organisations like Al-Qaeda and ISIS (Islamic State in Iraq and Syria) "know that they will never be able to defeat a nation as great and as strong as America," Obama said at a memorial service at the Pentagon, one of the sites attacked on Sept 11, 2001.

"So instead they try to terrorise, in the hopes that they can stoke enough fear that we turn on each other," said Obama, who observed a moment of silence Sunday at the Oval Office in honour of the nearly 3,000 people killed that day.

"And that's why it is so important today that we reaffirm our character as a nation (...) Our diversity, our patchwork heritage, is not a weakness. It is still and always will be one of our greatest strengths," he added.

"This is the America that was attacked that September morning. This is the America that we must remain true to."

In an indirect reference to the controversial anti-Muslim, anti-immigrant campaign rhetoric of Republican presidential candidate Donald Trump, Obama recalled that Americans were "a people drawn from every corner of the world, every colour, every religion, every background".

Obama, who is a Democrat, has repeatedly criticised Trump's statements, including the billionaire's proposal in the wake of last December's mass shooting attack in San Bernardino, California to ban Muslims from entering the United States.

In the 15 years since the Al-Qaeda suicide plane strikes on the Pentagon, New York's World Trade Centre, and in Pennsylvania, "the threat has evolved", the president said.

"With our stronger defences, terrorists often attempt attacks on a smaller but still deadly scale," he added, recalling the Boston marathon bombings, the nightclub massacre in Orlando in June and the carnage in San Bernardino.