Talks between Iran and six world powers in Istanbul on Tehran's nuclear programme, the first in 15 months, began Saturday in a "positive atmosphere," a European Union spokesman said.

"There is a positive atmosphere. ... There is a desire for substantive progress," Michael Mann, spokesman for EU foreign policy chief Catherine Ashton, told reporters.

"There is no disagreement yet," said Mann, calling the atmosphere "good and friendly."

But with mutual mistrust and tough talk on both sides major progress looks unlikely.

According to officials close to the negotiations between Iran and the United States, Russia, China, Britain, France and Germany (the P5+1), the most that can be hoped for is an agreement to hold more detailed discussions in a few weeks.

"It depends on what happens," said Mann, adding that if there is a certain extent of progress at the Istanbul talks, Western powers would opt to a second round.

But Mann said: "We have to do the first round of the talks first."

The two sides could have a new round of negotiations in May, said a source close to the talks.

Iran insists that this new round should take place in Baghdad.