VIENNA (Reuters) - Iran must drastically reduce its capacity to enrich uranium if it wants to reach a nuclear deal with six world powers that would lead to a gradual lifting of the crippling international sanctions on Teheran, a senior U.S. official said on Thursday.

Speaking about the number of centrifuges Iran operates to process uranium for use as nuclear fuel, an acceptable deal will require Teheran to maintain "a fraction of what they currently have", the official said at the start of a new round of negotiations between the Islamic Republic and the six powers.

The official, who spoke to reporters on condition of anonymity, rejected Iranian complaints that Washington was setting out "maximalist" positions in the talks: "We are putting down very reasonable positions."