Iran urged western powers Tuesday to resist any pressure from third parties not directly involved in negotiations over its nuclear activities, in a clear reference to Israeli-led influence.

Foreign Minister Mohammad Javad Zarif, quoted by the official IRNA news agency after meetings with EU foreign policy chief Catherine Ashton, also called for "seriousness and realism," in the next round of nuclear talks in Vienna next month.

After a second day of discussions with Ashton in Istanbul, Zarif said the P5+1 powers of Britain, China, France, Russia, the United States and Germany should avoid "illusions which might be exerted on them by some pressure groups and people outside the negotiations."

Iran has consistently denied it is seeking nuclear weapons but wants an independent atomic energy programme. Israel and lawmakers in the US Congress have repeatedly warned against lowering the pressure on Iran.

Zarif said reaching an agreement was "completely possible" and that the Islamic republic would take part in the talks with "seriousness, accuracy and without any rush."

But he condemned as "brutal obstacles" the sanctions imposed on Iran over its nuclear activities, pledging their removal was "the basis of our work" in seeking a comprehensive deal that would "preserve the rights and dignity of the nation."

The next round of talks on resolving Western concerns will take place in Vienna on June 16-20, the European Union announced on Tuesday.

Three days of such meetings two weeks ago made no "tangible progress," with a July 20 deadline for a conclusive agreement looming on the horizon and major issues still outstanding.

These reportedly include 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.

The differences during the last round in Vienna prevented a start being made on an early draft agreement.

Negotiators aim to nail down an exceedingly complex and lasting deal limiting Iran's atomic activities in exchange for a lifting of sanctions.

Failure could have calamitous consequences, potentially sparking conflict -- neither Israel nor the United States rules out military action -- and creating a nuclear arms race in the Middle East.

Negotiators could theoretically extend the July 20 deadline to win more time, but Presidents Barack Obama and Hassan Rouhani could struggle to keep sceptical and impatient US and Iranian hardliners at bay.

Iran and the P5+1, meanwhile, will hold expert-level talks on the sidelines of the IAEA's board of governors conference in Vienna starting next Monday, the Fars news agency reported.

A spokesman for the IAEA said the "meeting would probably last for four to five days".