In New York, a sixth bell has rung out, followed by another moment of silence, marking the exact moment the World Trade Center’s North Tower fell.

Addressing the crowd at the Pentagon’s commemorative ceremony, US president Donald Trump said, “in 25 years, a generation has passed since the terror attacks.”

“Yet time has not dimmed the honour of that day our country came face-to-face with true evil.”

“Today we renew the sacred oath that we first swore in their names a quarter of a century ago,” he said.

“We will never, ever forget. That’s why we fight today.”