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Talks to reach a preliminary agreement on Iran's nuclear programme are entering their final day, the BBC reports.

Foreign ministers from six world powers and their Iranian counterpart have been negotiating in Switzerland ahead of a self-imposed deadline.

US Secretary of State John Kerry said talks on Monday had produced "some light" but "tricky issues" remained.

Ministers want to restrict Iran's nuclear programme in exchange for relief from crippling sanctions.

Negotiators worked late into the night and are continuing talks this morning in an all out effort to meet the deadline.

The six global powers are closer than they have ever been to resolving the longstanding tensions over Iran's nuclear programme. Progress has been made on steps to curb and monitor Iran's production of enriched uranium, which can be used to make the core of a nuclear warhead.

But substantive differences remain. These include the pace of sanctions relief and the nature of restrictions on Iran's nuclear research and development.

If a broad framework agreement is reached by the end of the day, it would be used as the basis of a final accord. No-one has given a clear answer as to what would happen if it is not.