Osama bin Laden is warning that al Qaeda will kill any Americans it takes prisoner if the U.S. executes accused Sept. 11 co-conspirator Khalid Sheikh Mohammed.

"The day America will take such decision [to execute Mohammed and any others] it would have taken a decision to execute whoever we capture," bin Laden said in an audiotaped message aired Thursday on Al Jazeera. He said Mohammed’s execution would amount to a “death sentence” for Americans in the group’s custody.

In a military commission set up in 2008, Mohammed and four other accused Sept. 11 co-conspirators pled guilty and said they wanted to confess how they had committed the Sept. 11 attacks. The move was widely interpreted as challenging the court to put them to death and lend them martyrdom status.

But the Obama administration dropped the military charges against the five men in November when they announced they would move the trials to a federal court in New York City. That plan has since run into stiff political opposition and administration officials are considering sending the proceedings back to the military.

President Obama has cited the death penalty as a compelling reason to hold a civilian trial for Mohammed. “I don't think it will be offensive at all when he's convicted and when the death penalty is applied to him,” Obama said during his trip to China last fall.

Since Sept. 11, military commissions have convicted only three terrorists. Two were eventually released to their home countries, and one is now serving a life sentence at Guantanamo Bay. None received the death penalty. “The last time military tribunals resulted in the execution of foreign detainees was 1942, Nazi agents who were convicted for plotting domestic attacks,” said Adam Blickstein, spokesman for the left-leaning National Security Network.