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US and Iranian negotiators Thursday entered into a fourth day of tough talks towards a landmark nuclear deal, with the top diplomats of both countries both suuggesting signs of progress.

US Secretary of State John Kerry and his Iranian counterpart have been haggling in Switzerland since Monday as they seek to agree the outlines of this potentially historic agreement by March 31.

On Thursday, both said "progress" was being made.

The sooner Obama can bring back an agreement that meets US policy goals, the better. - Arms Control Association

"We're pushing some tough issues, but we made progress," Kerry told reporters in the Swiss city of Lausanne during a break from talks involving Iranian Foreign Minister Mohammad Javad Zarif all week.

They hope an accord, to be finalised by July, would convince the world after a standoff now in its 13th year that Iran won't build nuclear weapons under the guise of its civilian programme.

The agreement would involve Iran reducing its nuclear activities and ultra-tight inspections in exchange for relief from painful sanctions strangling its oil exports and economy.

Iran's Mohammad Javad Zarif said Wednesday that foreign ministers from the other five powers involved - Russia, China, Britain, France and Germany - were unlikely to join him and Kerry in Lausanne this week.

"When the solutions are found and we approach a deal, then all the foreign ministers of the negotiating parties should come," he said.

As a result, the negotiations will likely have to continue into next week or resume then after a break. Political directors from all countries involved joined the talks in the week.

On Tuesday the White House said the chances of such a deal are 50/50 "at best" with "some of the most difficult issues... yet to be resolved".

The mooted deal is highly complex, and negotiators have already missed two deadlines last July and November to clinch a deal despite numerous rounds of talks around the world.

Kerry cannot afford a new extension, however, experts say, with President Barack Obama's Republican opponents teeing up new sanctions legislation that would likely kill the entire process.

The Republicans and Israel's freshly re-elected Prime Minister Binyamin Netanyahu - whose country is widely understood to have nuclear weapons itself and which, unlike Iran, is not a signatory to the Nuclear Non-Proliferation Treaty - fear the agreement will not stop Iran getting the bomb.

"The sooner Obama can bring back an agreement that meets US policy goals, the better," Arms Control Association analyst Kelsey Davenport told AFP.