Talks between world powers and Iran over its nuclear programme remained at an impasse this week as the deadline for achieving a comprehensive agreement loomed.

US Secretary of State John Kerry, who participated in the negotiations for two days, said on Tuesday that despite some progress, "very real gaps" remained.

The talks in Vienna between Iran and the P5+1 group of nations - the six major nuclear powers - have been ongoing for two weeks. The chief negotiator for the Western powers, EU foreign affairs chief Baroness Ashton, was joined earlier this week by Mr Kerry and other foreign ministers in an attempt to achieve a breakthrough before the July 20 deadline. The main point of disagreement is over the number of centrifuges that Iran will be allowed to keep to enrich uranium.

Iran's chief negotiator, Foreign Minister Muhammad Javad Zarif, proposed to the New York Times that Iran would agree to freeze its current enrichment capacity for seven years. This proposal is unlikely to be accepted.

It is not clear what will happen if an agreement is not signed before the deadline and whether the interim deal signed in November in Geneva is to be extended. A senior Israeli diplomat said that "the differences between the sides seem at this point insurmountable."