US Secret Service releases never-before-seen photos of 9/11 terror attacks 20 years on

The US Secret Service is releasing never-before-seen photos of the terror attacks on September 11, 2001 as the 20th anniversary of the attack approaches.

Over the past two days, the Secret Service has been sharing photos of the day to its social media accounts with the hashtag #NeverForget.

One of the photos shows the twin towers of the World Trade Centre after both planes had struck.

The never-before-seen photo, which was taken by a Secret Service employee, shows smoke and fire billowing out of the buildings.

Another photo shows the aftermath of the attacks, where the streets of New York were filled with an orange haze.

"Ground Zero on Sept. 11, 2001, after the Towers collapse, taken by a Secret Service employee," the caption says.

Other photos show armored limousines parked at the former Secret Service New York Field Office that were damaged and covered in debris from the attack, while another shows one of the buildings collapsing.

The 11th of September (Saturday NZT) marks the 20th anniversary of the attacks, where 19 terrorists hijacked four planes and successfully flew three of them into major US landmarks.

American Airlines Flight 11 and United Airlines Flight 175 were flown into the North and South Towers of New York's World Trade Centre on that morning. Not long after, American Airlines Flight 77 was flown into Virginia's Pentagon building, and United Airlines Flight 93 crashed near Shanksville, Pennsylvania. 

Less than half an hour after the crashes, both Twin Towers had collapsed.

Around 2996 people were killed in the attack and more than 6000 injured.

Current US President Joe Biden is expected to mark the day by visiting all three sites where planes crashed.

The White House said Biden and his wife Jill will visit New York City, Shanksville, Pennsylvania, and the Pentagon. Vice President Kamala Harris and her partner will also visit Shanksville for a separate event, before joining the Bidens at the Pentagon, CBS News reported.