Meeting on the sidelines of a major United Nations gathering in New York, President Rouhani spoke of the need to take significant steps forward if a comprehensive agrement to curtail its nuclear program - in exchange for the lifting of economic sanctions - is to be agreed by the current November 24 deadline.

"The remaining time for reaching an agreement is extremely short. Progress that has been witnessed in the last few days has been extremely slow," Rouhani told reporters Friday as talks concluded.

Those sentiments were echoed by a senior US State Department official, who said "the gaps are still serious" when it came to the positions of both sides following more than a week of discussions.

"We do not have an understanding on all the major issues, we have some understandings that are helpful to move this process forward and we have an enormous number of details still to work through," the official, speaking under condition of anonymity, told reporters. "We still have some very, very difficult understandings yet to reach."

Russian Foreign Minister Sergei Lavrov was more upbeat, saying that both sides were interested in resolving the remaining "small but extremely important issues."

US Secretary of State John Kerry told reporters the breakthrough interim deal struck in Geneva last November, in which Iran halted parts of its nuclear program in return for some sanctions relief, had "made the world safer."

Iran and the five permanent members of the UN Security Council - Britain, China, France, Russia and the United States - plus Germany, collectively known as the P5+1 group, had already extended their July deadline to the end of November.

Such a deal would resolve a decades-long standoff between Iran and Western nations, who fear that Tehran is developing nuclear weapons. Iran has always insisted its nuclear program was for civilian and scientific purposes.

Uranium enrichment

One of the key sticking points in the negotiations is uranium enrichment. The process is used to make reactor fuel for power plants; taken a step further, however, it can also be used to make a nuclear bomb.

The number of enrichment centrifuges that Iran would be allowed to keep, in what state they could operate and for how long Iran would have to reduce its enrichment capacity are all points of continued debate between the parties.

Another topic of contention has been the speed with which sanctions against Tehran would be lifted. No date or venue has yet been set for another round of talks.

se/glb (AFP, Reuters, AP)