Talks on implementing last month's nuclear deal with Iran are proving to be a hard slog, diplomats said Thursday on a fourth day of discussions with no end in sight.
Participants in Vienna said there were differences of opinion about sequencing Iran's promised nuclear freeze and the easing of sanctions agreed in last month's landmark deal in Geneva.
They insisted however that the discussions were not any harder than expected and that the differences would eventually be ironed out -- although not by the end of this week.
"There are definite differences of opinion on the interpretation (of the Geneva text). Not that I am saying these are insurmountable but both sides are looking to negotiate the most robust deal they can," one Western diplomat involved in the talks told AFP.
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"What this means, and this is not a surprise, is that we will not get this resolved by the end of this week.... They are going to have to get together more frequently than they thought."
A second said: "I think it is too early to say whether these talks are proving more difficult than we had imagined. These are very technical talks involving experts and there are a lot of points to work out."
The discussions involve experts from Iran and the P5+1 world powers -- the United States, China, Russia, Britain, France and Germany -- plus the International Atomic Energy Agency, which will verify Iran's freeze.
Under the Geneva deal struck on November 24, Iran will roll back parts of its nuclear programme for six months in exchange for minor sanctions relief while a long-term "comprehensive" accord is hammered out.
The start gun on the six months has not yet sounded, however. Iran's envoy to the IAEA said it should begin in late December or early January but analysts say this may be ambitious.