Iranian Foreign Minister Mohammad Javad Zarif, center, attends talks on Iran"s nuclear program in Geneva on Friday Nov. 22, 2013
Media playback is unsupported on your device
Video

'Heavyweights' joining Iran nuclear talks

The UK's William Hague, US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov are said to be preparing to join negotiations with Iran over its nuclear programme.

As the talks moved into a third day, Iran said there were "serious issues" to be resolved, while one Western diplomat cited "considerable gaps".

Jeremy Bowen reports from Geneva.