An audiotape said to be of Osama bin Laden threatens to kill Americans if the alleged mastermind of the September 11 terrorist attacks is executed.
Al-Jazeera news channel broadcast the tape reportedly from the al-Qaeda leader talking about Khalid Sheikh Mohammed and four other suspects.
They are due to be tried in New York for the 2001 attacks, the BBC reports.
The tape said that if the US made the decision to execute, al-Qaeda would also "execute" anyone it captured.
The Pentagon says Mr Mohammed has admitted to being responsible for the attacks in New York and Washington.
Following his capture in Pakistan in 2003, Mr Mohammed was held at a CIA secret prison, where he was subjected to harsh interrogation techniques and a practice known as "waterboarding", which simulates drowning, until he was moved to Guantanamo Bay in 2006.
In the tape broadcast on Thursday, the speaker also accused US President Barack Obama of following the footsteps of his predecessor.