"Intense and difficult negotiations are under way and it is not clear whether we will reach an agreement tonight," Fars news agency quoted Araqchi as saying.
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"The dispute is over the wording."
The two sides are hoping to reach a long elusive deal under which Iran would freeze or curb parts of its controversial nuclear program in exchange for some relief from crippling sanctions.
The arrival of the top diplomats from the so called P5+1 – Britain, China, France, Russia and the United States plus Germany – on Saturday raised hopes after three days of intense negotiations among top officials that a breakthrough was in sight.
"The foreign ministers of the six countries are going to have a meeting together...We will decide about the continuation of the negotiations after" this meeting, Araqchi told Mehr.
Negotiators in Geneva have expressed the hope of that an agreement will be reached, while also admitting that difficulties remain.
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