In a new audio tape, a voice believed to be Abu Hamza al-Muhajir calls on supporters to kidnap Westerners during the current holy month of Ramadan.
Some militant Muslims believe those who die in the pursuit of jihad during Ramadan are afforded special status in the afterlife.
Muhajir said the hostages could be exchanged for blind Egyptian Muslim cleric Sheik Omar Abdel Rahman, convicted in 1995 of plotting to blow up New York landmarks and now in a federal medical prison in Springfield, Mo.
Muhajir urged the mujahedin "to capture some of the Roman dogs so as to secure his release from the darkness of his prison," The Sunday Times of London reported.
U.S. officials had no immediate comment.