The US Supreme Court on Monday declined to hear appeals from Khalid Sheikh Mohammed and two co-defendants who wanted the court to enforce 2024 plea agreements that would have spared them the death penalty.

The justices turned the appeals away in an unsigned order without giving reasons, leaving in place a lower court decision that upheld the government's withdrawal of the deals. Justice Brett Kavanaugh did not take part in the case.

Mohammed is accused of being the main planner of the 11 September 2001 attacks, in which hijacked planes brought down the World Trade Center towers and struck the Pentagon, killing nearly 3,000 people. He has been held at the US military prison at Guantanamo Bay, Cuba, since 2006.

The other two men are Mustafa Ahmed al-Hawsawi and Walid bin Attash, who are also held at Guantanamo.

Under the 2024 agreements, approved by the military commission's convening authority, the three would have pleaded guilty to all charges and received life sentences instead of facing execution. Then defence secretary Lloyd Austin withdrew the deals two days later, saying he had the authority to do so.

A military judge ruled that Austin had acted unlawfully, but a federal appeals court in Washington later sided with the government, prompting the men to ask the Supreme Court to step in.

A military commission trial is scheduled for June 2028, where the death penalty remains a possible sentence. The case has been slowed for years by disputes over evidence the men were subjected to torture while in CIA custody.