The alleged mastermind behind the September 11 terror attacks will go on military trial in 2028, a judge has ruled.

Khalid Sheikh Mohammed has been held at Guantanamo Bay, the US detention camp in Cuba, since 2006.

The 61-year-old, of Pakistan, is accused of orchestrating the 9/11 atrocities almost 25 years ago, which saw al-Qaeda hijackers fly planes into the World Trade Centre's two towers in New York and the Pentagon in Washington.

Another plane crashed in a field in Pennsylvania after passengers fought to regain control of the aircraft, which was targeting another government building in DC.

Across all four planes, nearly 3,000 people were killed.

Mohammed was captured in Pakistan in 2003 and charged with war crimes and murder in 2008.

His two decades in captivity in Guantanamo have seen planned trial dates come and go, including due to plea deal negotiations and the COVID pandemic.

A Senate report in 2014 also found he had been subjected to waterboarding 183 times. Defence lawyers have argued that the holdup comes from what they say are the government's efforts to hide details of torture.

Read more: 9/11 in pictures

He will be prosecuted alongside others suspected of conspiring in the attacks: Walid bin Attash, Ali Abdul Aziz Ali and Mustafa Ahmed al-Hawsawi.

NBC News, Sky's US partner network, reports the new June 2028 start date was set by military commissions judge Michael Schrama, who rejected a January 2027 window proposed by prosecutors.

It comes after an appeals court last summer threw out a potential agreement that would have allowed Mohammed to plead guilty in exchange for avoiding the risk of the death penalty.

(c) Sky News 2026: Alleged 9/11 mastermind gets military trial date