Ayman al-Zawahiri called for the abductions on a video more than 2 hours long posted Wednesday on jihadist websites, CNN reported Saturday.
In the video, Zawahiri cited al-Qaida's success in the August 2011 abduction of American aid worker Warren Weinstein in Pakistan. Zawahiri said the kidnapping of more Americans could be used to force the release of Sheik Omar Abdel-Rahman.
Abdel-Rahman, known as the "blind sheik," was convicted in 1995 of conspiracy in the New York bombing and is serving a life sentence at a federal prison in North Carolina.
Al-Qaida released a video in May of Weinstein, 71, pleading with President Barack Obama to meet the group's demands, which include the release of Abdel-Rahman.
U.S. officials have said they will not bargain with al-Qaida.
Zawahiri assumed the leadership of the terrorist organization after the death of Osama bin Laden.