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

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.

'More light'

Iran insists its nuclear programme is for peaceful purposes, but world powers are worried about the country developing nuclear weapons.

They want to keep Iran at least one year away from being able to produce enough fuel for a single weapon.

The final hours of negotiation in Lausanne are taking place between foreign ministers from the so-called P5+1 - comprising the US, UK, France, China, Russia and Germany - and Iranian Foreign Minister Mohammad Javad Zarif. EU foreign policy chief Federica Mogherini is also present.

Kerry said there had been "a little more light" on Monday, "but there are still some tricky issues. Everyone knows the meaning of tomorrow".

China's Foreign Minister Wang Yi said on Monday that the "marathon-like" negotiations had entered the final stage and that he was "cautiously optimistic".

The differences between the parties were narrowing, he said.

At the scene: Barbara Plett, BBC News, Lausanne

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 here has given a clear answer as to what would happen if it is not.

Ministers are aiming to agree on a political framework agreement by Tuesday night that would lead to a final and comprehensive accord by 30 June.

Senior Iranian negotiator Abbas Araqchi told Iranian TV that he was "hopeful" about a deal, but that ministers were not in a position to say whether they were close to resolving all of the issues.

Iranian and Western officials have said that a deal is possible, but after almost 18 months of negotiations several sticking points remain.

Three of the major outstanding issues are:

• Length of restrictions - Iran's nuclear activities would be strictly limited for at least 10 years. After that, Iran wants all limits to be lifted. The P5+1 says they should be removed progressively over the following five years
• Sanctions relief - Iran wants the UN sanctions suspended soon after an agreement. The P5+1 says they should be eased in a phased manner, with restrictions on imports of nuclear-related technology remaining for years
• Non-compliance - The US and its European allies want a mechanism that would allow suspended UN sanctions to be put back into effect rapidly if Iran reneges on a deal. Russia reportedly accepts this, but wants to ensure its Security Council veto rights are protected

"We must take precautions to ensure that what happens after the 10 years expire really is verifiable and transparent," said German Foreign Minister Frank-Walter Steinmeier. "We cannot allow there to be a really explosive development after 10 years."

Another point of contention is Iran's desire to be able to develop advanced centrifuges, which could enrich uranium faster and in greater quantities. While enriched uranium is used as fuel for nuclear reactors, it can also be used to make nuclear bombs.

Adding to the list of issues to be resolved, Iran's lead negotiator has ruled out sending its existing stockpile of nuclear fuel abroad, one of the steps demanded by the P5+1.

Meanwhile, Israel's Prime Minister Benjamin Netanyahu reiterated his opposition to a deal, saying it would send the message "that Iran stands to gain by its aggression".