'We must keep US free'

President Barack Obama yesterday urged Americans to remain united in the face of terrorist attacks, in a barely-veiled jab at Republican White House nominee Donald Trump 15 years after 9/11.

"In the face of terrorism, how we respond matters," Obama said in his weekly radio and online address, delivered on the eve of the 15th anniversary of the September 11 terror attacks in the United States.

"We cannot give in to those who would divide us. We cannot react in ways that erode the fabric of our society," he added.

"Because it's our diversity, our welcoming of all talent, our treating of everybody fairly-no matter their race, gender, ethnicity, or faith-that's part of what makes our country great. It's what makes us resilient," Obama said.

"And if we stay true to those values, we'll uphold the legacy of those we've lost, and keep our nation strong and free."

The Al-Qaeda hijackings of September 11, 2001 -- the first foreign attack on the US mainland in nearly two centuries -- ruptured a sense of safety and plunged the West into wars still being fought today.

More than 2,750 people were killed when two passenger jets destroyed the Twin Towers, the symbol of New York's financial wealth and confidence. Another jet slammed into the Pentagon, and a fourth jet crashed in a field in Pennsylvania after those on board tried to overpower the hijackers.

"We will destroy them. And we'll keep doing everything in our power to protect our homeland," Obama said.