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.

"Our diversity, our patchwork heritage, is not a weakness. It is still and always will be one of our greatest strengths," Obama said at a remembrance service at the Pentagon, one of the sites attacked on 9/11.

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