During a ceremony at 15th anniversary 9/11 attcks at the Pentagon he said, "together, there is nothing we Americans cannot overcome."
At the Pentagon, a trumpet played as President Barack Obama took part in a wreath-laying ceremony.
"It is so important today that we reaffirm our character as a nation, of people drawn from every corner of the world, every colour, every religion, every background," New York Daily News quoted Obama as saying.
"Bound by a creed as old as our founder ... Out of many, we are one. For we know that our diversity, our patchwork heritage, is not a weakness, it is still and always will be one of our greatest strengths," Obama said.
Obama also vowed that the US will never bow to the wishes of terrorists who, through their attacks, hope to change the way Americans live.
"In the end, the most enduring memorial to those we lost is ensuring the America that we continue to be," Obama noted, adding, "That we stay true to ourselves. That we stay true to what's best in us. That we do not let others divide us."
Meanwhile, Donald Trump released a statement on Facebook page and offered his condolences toy he victims.
Hillary Clinton, who had fainted at a public event due to "overheating" and was later diagnosed with pneumonia, said the attack was the closest thing to "hell" she had ever seen.
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