VIENNA (REUTERS) - Iran and six major powers are ready to start drafting a long-term agreement on curbing Teheran's nuclear programme in exchange for sanctions relief but it is unclear if they will succeed in overcoming disagreements, a United States official said on Wednesday.

"Now we are set to start drafting," the senior administration official told reporters on condition of anonymity at the end of a two-day round of talks in Vienna between Iran and the United States, Britain, France, Germany, Russia and China.

"At this point we don't know if we'll be successful in bridging those gaps," the official said.