A unilateral U.S. oil embargo on Iran is expected to remain in place even if a long-term nuclear agreement between Tehran and six world powers is reached that includes an easing of international sanctions, a U.S. official said on Thursday.

The embargo pre-dates the decade-long nuclear dispute with Iran. Washington cut off diplomatic ties with Tehran during a hostage crisis shortly after the 1979 Islamic Revolution and began imposing sanctions on the Iranian government around the same time.

"The American domestic oil embargo is expected to remain in place even if a comprehensive agreement is reached," the U.S. official told Reuters on condition of anonymity. The official spoke after Iran and the six powers agreed on an agenda and schedule for months of talks aimed at ending the dispute over Tehran's nuclear ambitions.