Hassan Rouhani, who has placed his credibility on ending more than a decade of international concern that the Islamic republic is developing a nuclear bomb, also acknowledged sanctions would not be lifted immediately under the accord, which is due by June 30.
He made the remarks in Tehran a day after Russia's senior negotiator spoke of a "very worrying" slowdown in progress in painstaking diplomacy which has lasted 21 months.
With the finishing line in sight, Rouhani, speaking at a press conference marking two years since his unexpected election victory, accused the other side of agreeing terms in one meeting only to try and improve on them during later discussions.
"If the other party respects the agreed framework and does not add other demands, the differences can be resolved but if they choose the path of haggling then it can prolong the negotiations," he added.
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Iran and the P5+1 group (Britain, China, France, Russia and the US plus Germany) agreed the outlines of the nuclear deal on April 2 after intensive talks went past a March 31 deadline.
In Washington yesterday, State Department spokesman Jeffrey Rathke admitted the talks were proving "complicated", but the US believes a deal can still be reached by the end of June.
With deputy foreign ministers and technical experts currently meeting in Vienna in the run up to the proposed final talks in late June, Russia's senior negotiator spoke Friday of slow progress.
"There is very little time before the deadline and we urgently need to enter the final stage," said Sergei Ryabkov, a deputy foreign minister.
Alluding to those concerns, Rouhani said there are still "many differences over details that must be addressed" under the deal, which aims to guarantee that Iran's nuclear activities are peaceful, but he remained "hopeful" of an agreement.