ISTANBUL - Iran’s delegation to crunch talks with world powers on its nuclear programme Saturday turned down a US request for what would have been a rare bilateral meeting on the sidelines, a source told. “Their request was presented numerous times but Iran has refused,” said the source close to the Islamic republic’s team at the talks with six world powers in Istanbul, which diplomats said were nonetheless going well. Earlier diplomats at the talks had said that the US delegation led by Wendy Sherman, undersecretary for political affairs, had let Iran know she was ready for face-to-face discussions with her counterpart Saeed Jalili.

The message was passed to the Iranians by Catherine Ashton, the EU foreign policy chief, on the sidelines of the discussions between Iran and the United States, China, Russia, Britain, France and Germany, envoys said.

The last time that the US and Iran held bilateral talks about the nuclear issue was on the sidelines of a similar meeting in Geneva on October 1, 2009, when William Burns, the senior official leading Washington’s delegation, met Jalili.

Despite the refusal, diplomats said that a positive new Iranian attitude in Istanbul raised the prospect for a second round of more in-depth discussions, with one envoy saying this should take place in less than four weeks.

Western powers suspect Iran’s nuclear programme is a cover for a secret atomic weapons drive, a charge Tehran vigorously denies. The UN Security Council has passed four rounds of sanctions on the country.

The main concern of the international community, particularly for Iran’s arch foe Israel, is Tehran’s growing capacity to enrich uranium, which can be used for peaceful uses but when purified further for a nuclear weapon.