Iran and major world powers are gathering in Istanbul, Turkey, for a new round of talks on Tehran's controversial nuclear programme. Negotiators from China, France, Germany, Russia, the UK and US will meet their Iranian counterparts today and tomorrow. The talks will be chaired by European Union foreign policy Chief Baroness Catherine Ashton. Ahead of the summit, US state department spokesman Mark Toner said Washington was not expecting any big breakthroughs. The UN has imposed four rounds of sanctions on Iran in recent years for not fully disclosing its programme. The West suspects Iran aims to build nuclear weapons, which Tehran denies. Iran insists its uranium enrichment programme is peaceful and complies with international law.