Iran and major nations have a "historic opportunity" to settle their decade-old nuclear dispute, but requiring the Islamic state to stop higher-grade uranium enrichment would be discriminatory, Tehran's former chief nuclear negotiator said.

According to The Jerusalem Post, Hossein Mousavian, now a visiting scholar at Princeton University in the United States, voiced optimism before next month's talks between Iran and the six major powers following a first meeting in Istanbul earlier this month.

They should set out their respective "red lines" regarding Iran's nuclear program and negotiate on the basis of those when they meet in the Iraqi capital on May 23, he told Reuters.