Iran nuclear talks explained - in 60 seconds
Iran is holding three days of talks in Vienna with six world powers in an attempt to agree on limiting its nuclear programme.
The meeting meeting with the "P5+1" - the US, UK, France, China and Russia plus Germany - seeks to build on an interim deal signed in November.
That saw Iran agree to curb its uranium enrichment in return for partial sanctions relief.
But both Iran and the US have downplayed hopes of a quick breakthrough.
BBC News explains the background to the talks - in 60 seconds.
Video produced by Michael Hirst
- From the section Middle East