"We have reached an agreement on some questions, but on others there are still disagreements.... There are differences of opinion within the P5+1 group" of world powers, Zarif has said.

The Islamic Republic's deputy foreign minister, Abbas Araqchi, also stated "the issues are serious and there is still a gap in stances", and that the talks would probably end later in the day and be resumed at a later date.

Araqchi also added that his counterparts from the six powers "need constant coordination and consultation in order to determine (their) stances."

British Foreign Secretary William Hague earlier said the talks have achieved "very good progress" but much more needed to be agreed and it was unclear if this would happen by the end of the day.

"We are very conscious of the fact that real momentum has built up in these negotiations," he told reporters. "So we have to do everything we can to seize the moment."

Foreign ministers from all five permanent UN Security Council members - Britain, China, France, Russia and the United States - and Germany were due to take part in Saturday's talks with Zarif.

RA/