Technical experts from Iran and major world powers gathered Tuesday in Istanbul for closed door talks on Tehran's disputed nuclear programme, a European diplomat said.

The meeting between nuclear physics experts from Iran and the major powers known as P5+1 -- the five permanent members of the UN Security Council and Germany -- takes place without the participation of diplomats, the source said.

This technical level resumption of talks over Iran's controversial nuclear aims was set after the last summit between the P5+1 and Iran in June in Moscow.

The downgraded round of talks comes at a time when Iran hardened the tone of its criticism against the Western countries for dragging out the negotiations.

Iranian foreign ministry spokesman Ramin Mehmanparast told a weekly briefing that if the powers ignored Iran's nuclear "rights" and failed to bargain on equal terms, the negotiations could lead to an "impasse".

"All that can reinforce the idea that there is a desire to drag out the negotiations or prevent their success," he said.

Iran is insisting it has a "right" to uranium enrichment under the nuclear Non-Proliferation Treaty, which should be recognised by the P5+1. It also wants Western sanctions punishing its economy to be eased.

The world powers instead are pushing for an immediate end to Iran's enriching uranium to 20 percent purity -- just a technical step short of the 90 percent needed to make nuclear bombs -- and to ship out its existing 20-percent stock and close a fortified underground enrichment facility in Fordo.