By Louis Charbonneau and Fredrik Dahl
VIENNA (Reuters) - A senior U.S. official on Tuesday cautioned against excessive optimism over six world powers' nuclear talks with Iran but said disputes could be overcome and a deal reached by their self-imposed July 20 deadline.
The official noted some media reports about the negotiations that have wrongly implied a deal called a "comprehensive plan of action" between Iran and the six powers was a virtual certainty.
Iran joined talks on its nuclear dispute with big powers after President Hassan Rouhani was elected last June. They yielded an interim deal in November, easing fears of a wider Middle East war and paving the way for the current talks
"I've read a lot of the optimism you've written ... it's gotten way out of control," the official told reporters on condition of anonymity ahead of the fourth round of negotiations. "There are some very significant gaps (but) we can get to a resolution, I believe."
The official said delegations from Iran, United States, Britain, France, Germany, Russia and China would begin drafting an agreement this week that aims to cover specific curbs on Tehran's nuclear program in exchange for the gradual lifting of sanctions against Iran.
"Just because we will be drafting it certainly does not mean an agreement is imminent or that we are certain to eventually get to a resolution of these issues," the official added.
Diplomats close to the talks say the scope of Iran's future uranium enrichment activity and research and development of the equipment used to purify uranium remain a difficult sticking point in the negotiations.
Enriched uranium can be used as fuel in nuclear power plants or in weapons if purified to a high enough level. Tehran rejects allegations from Western powers that it is seeking a nuclear weapons capability under cover of a civilian energy program.
Asked about the possible impact on the Iran talks of the Ukraine crisis, which has led to U.S. and European sanctions against Russian individuals and firms, the official said it was "not discernible."
"Our Russian colleagues have focused in on this negotiation with the same seriousness of purpose that everyone else at the table has," the official said.
(Reporting By Louis Charbonneau and Fredrik Dahl; Editing by Tom Heneghan)