By Fredrik Dahl
VIENNA (Reuters) - Britain urged Iran to show more flexibility in nuclear talks due to start on Tuesday while Tehran insisted its rights must be respected, highlighting gaps standing in the way of an historic deal by a Nov. 24 deadline.
U.S. Secretary of State John Kerry declined to make any predictions for what he called a "critical week", when negotiators from Iran and six world powers push to end a 12-year dispute over Tehran's nuclear programme and dispel fears of a new war in the Middle East.
After nearly a year of diplomacy, they aim to reach a comprehensive settlement at the talks in Vienna that would curb Iran's atomic activities in return for a phasing out of sanctions that have severely hurt its oil-dependent economy.
However, Iranian and Western officials have said next Monday's self-imposed deadline is unlikely to be met, and that an extension is the most likely scenario. They say it is possible to agree the outline of a future accord, but that it would take months to work out the details.
"This is a very critical week," Kerry said on a visit to London. "We hope we can get there but we can't make any predictions."
His British counterpart, Foreign Secretary Philip Hammond, expressed cautious optimism. "I believe a deal can be done," he said after meeting Kerry. "But we will not do a bad deal. These negotiations are extremely tough and Iran needs to show more flexibility if we are to succeed."
The outcome of the negotiations could have far-reaching implications, in the wider Middle East as well as domestically in the United States and Iran, where hardliners are sceptical of a rapprochement between the two arch foes.
The six states - France, China, Russia and Germany, as well as the United States and Britain - want Iran to scale back its capacity to refine uranium so that it would take much longer to produce fissile material for a bomb if it wanted to.
Tehran says it is enriching uranium only to make fuel for nuclear power plants and that this is its sovereign right.
Iran's official IRNA news agency quoted Foreign Minister Mohammad Javad Zarif as saying after arriving in Vienna: "We are here to find a solution that respects the Iranian nation's rights and removes the legitimate concerns of the international community."
He will meet former European Union foreign policy chief Catherine Ashton, who is coordinating the negotiations, over lunch before the start of the formal talks in the afternoon.
(Reporting by Parisa Hafezi in Ankara, and William James and Jonathan Allen in London; writing by Fredrik Dahl; editing by David Stamp)