Iran says it will be "impossible" to reach a deal with Western powers over its nuclear programme by tomorrow's deadline.

Negotiations in Vienna have been taking place with US Secretary of State John Kerry and five other countries who want assurances that Iran is not planning to build nuclear weapons.

The lifting of international sanctions against Iran is possible if Tehran curtails its nuclear programme.

Iran has insisted their programmes are peaceful and that they are not developing nuclear weapons.

Iranian representatives have said that an extension of the talks "is an option on the table", and will be discussed when the deadline passes.

One negotiator said that both sides are deadlocked on several issues, such as "the number of centrifuges, enrichment capacity and the timeframe of lifting sanctions".

France, Britain, Russia, Germany, China and the US are involved in the negotiations with Iran.