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By John Irish and Lesley Wroughton VIENNA (Reuters) - U.S. Secretary of State John Kerry said on Sunday major differences persist between Iran and six world powers negotiating on Tehran's nuclear program, with a week to go before a deadline for a deal. The United States, Britain, France, Germany, Russia and China want Iran to reduce its nuclear fuel-making capacity to deny it any means of quickly producing atom bombs. In exchange, international sanctions that have crippled the large OPEC member's oil-dependent economy would gradually be lifted. Iran says it is enriching uranium for peaceful energy purposes only and wants the sanctions removed swiftly.