Nation will never forget 9/11: Obama

President Barack Obama has praised America's diversity and urged Americans not to let their enemies divide them at a September 11 memorial.

President Barack Obama says the US will never forget those killed in the September 11 attacks. (AAP)

President Barack Obama says the nation will never forget the lives of those killed in the September 11 attacks.

Obama said at a Pentagon memorial service that he is inspired by the resilience of the victims' families.

He quoted Scripture: "Let not steadfast love and faithfulness forsake you. Write them on the table of your heart."

Obama also praised America's diversity and urged Americans not to let their enemies divide them.

He called the day "difficult" but one that 'reveals the love and faithfulness in your hearts and in the heart of our nation".

At the White House, Obama observed a moment of silence at 8:46am - the time the first hijacked airplane hit the north tower of New York City's World Trade Center.