Osama bin Laden threatened al Qaeda would kill any Americans it takes prisoner if accused Sept. 11 mastermind Khalid Sheikh Mohammed is put to death, according to an audiotape aired on al Jazeera on Thursday.
Senior U.S. officials may recommend that Mohammed, who was being held at the U.S. military prison at Guantanamo Bay, Cuba, be prosecuted in a military trial, officials said in March.
Such a decision, after intense political pressure, would reverse plans to hold criminal trials for Mohammed and four accused co-conspirators in a lower Manhattan court.
"This message is about our prisoners who you are holding," bin Laden said in the tape, complaining that U.S. President Barack Obama was still following "his predecessor's steps" on many issues including on al Qaeda detainees such as Mohammed.
"The White House has expressed its desire to execute him. When America makes this decision, it will have made a decision to execute whoever of you is held prisoner by us," bin Laden said in the tape recording, whose authenticity could not be immediately confirmed.
The White House had no immediate comment. A U.S. counter terrorism official dismissed as absurd al Qaeda leader bin Laden's threat to execute any Americans in captivity.
"It's the height of absurdity for anyone associated with al Qaeda to even suggest that now, at long last, they're going to start treating captives badly," said the official, who spoke on condition of anonymity.
"They may have forgotten Danny Pearl and all the others they've slaughtered, but we haven't."
The threat could signal an increased kidnapping risk targeting U.S. citizens that could last throughout the trial in the United States, the U.S.-based IntelCenter, which monitors jihadist propaganda, said in a note.
"The threat of kidnappings will increase further as the trial begins. Attempts to kidnap Americans would not be limited to core al Qaeda," the note said.
"The group's regional arms such as al Qaeda in the Arabian Peninsula (AQAP) and al Qaeda in the Land of the Islamic Maghreb, which has been aggressively targeting Westerners for kidnapping in North Africa, may follow through on bin Laden's threat," it added.
Mohammed has claimed responsibility for organising the Sept. 11, 2001, attacks on the United States and bombs in Indonesia, Kenya and elsewhere, and if convicted of murder, conspiracy, terrorism and other charges, could face the death penalty.
"It is fair to treat each other the same. War is a back-and-forth," bin Laden said.
"The White House politicians were and still are oppressing us, particularly in providing support to the Israelis and occupying our land in Palestine. They were thinking that America was safe behind oceans from the anger of the oppressed until the reaction was strong in your house on the 11th."
When Obama took office in January 2009, he set a one-year deadline to close the Guantanamo facility but political and diplomatic complications have forced a delay.
The administration plans to prosecute almost three dozen terrorism suspects held at Guantanamo Bay but has not announced where all the trials will be held and whether they will be criminal or military.