A report by the London-based Oxford Research Group said a U.S. attack on Iran's nuclear facilities would require more than 100 B-2 and B-1B strategic bombers. The report said the U.S. strike, which would require up to five days, must destroy about 20 key nuclear and military targets.

"An attack on Iranian nuclear infrastructure would signal the start of a protracted military confrontation that would probably grow to involve Iraq, Israel and Lebanon, as well as the USA and Iran," the report, released in February 2006, said.

The report said the U.S. attack must take place before 2007 and include the Bushehr nuclear reactor. Bushehr was scheduled to begin full-scale operations over the next few months.