President Hassan Rouhani said Monday he remains hopeful of a comprehensive agreement with world powers despite "difficulties" in negotiations on Iran's disputed nuclear drive.

No tangible progress was made in the latest round of talks that ended on Friday in Vienna, with time running out to clinch a deal by the agreed deadline of July 20.

In his first comments on the latest round, Rouhani said it was in the interest of both Iran and its interlocutors, the five UN Security Council permanent members plus Germany -- known as the P5+1 group -- to strike an agreement.

"In spite of the existing difficulties, God willing, the negotiations with the P5+1 group will finally result in a deal," Rouhani said, quoted by state news agency IRNA.

Diplomats have warned that the process of reining in Iran's controversial nuclear programme in exchange for relief from sanctions would be tough.

Negotiators had been expected to start work on drafting the text of an agreement at the three-day meeting last week.

But Iran's lead negotiator Abbas Araqchi said "the gaps were too large," and a senior US official spoke of "a very slow and difficult process".

According to media reports, among those gaps are the scope of Iran's enrichment of uranium, which if further purified could be used to trigger a nuclear explosion, and its unfinished Arak research reactor, whose by-product waste could provide an alternative route to an atomic bomb.

Iran insists its activities are aimed at civilian uses of nuclear technology, while Western powers suspect its drive masks military objectives.

Iran's refusal to widen the scope of the talks to cover its development of ballistic missiles also reportedly caused a rift.

Negotiators from Iran and the P5+1 are expected to reconvene in Vienna to try to narrow the gaps, but no date for a new round has been finalised.