US Secretary of State John Kerry and his Iranian counterpart Thursday hailed "progress" in marathon nuclear talks but Washington denied a draft deal was already being circulated.

A European negotiator said meanwhile that Iran and six major powers -- the US, China, Russia, Britain, France and Germany -- remained "pretty far from a deal" ahead of a March 31 deadline to agree the outlines of an agreement.

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

"We are advancing very well but there's still a lot of work to be done," Zarif told Iran's state news agency IRNA.

But US officials denied as "erroneous" press reports Thursday that a draft document was already being worked on.

Iran's deputy foreign minister Abbas Araqchi also seemed to reject the reports, telling Iranian state television that "for the time being, there is no deal."

"The Iranians go back, go forward, it changes every day," the European diplomat said. "There is nothing agreed in any format".

- Standoff -

The mooted agreement, due to be finalised by July, is aimed at convincing 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 highly complex deal would likely involve Iran reducing in scope its nuclear activities, allowing ultra-tight inspections, exporting nuclear material and limiting development of new nuclear machinery.

In exchange Iran would be granted staggered relief from the mountain of painful sanctions that have strangled its oil exports and hammered its economy.

Tehran, which denies wanting nuclear weapons, also wants to expand its activities in order to fuel nuclear power stations and meet its energy needs.

- No breakthrough this week -

The European diplomat said he did not expect a breakthrough by Friday, the tentative scheduled end of this round talks, meaning that negotiators will likely have to return next week.

Araqchi also hinted that could happen, saying Tehran was "prepared to prolong the negotiations if necessary."

Zarif meanwhile said Wednesday that the arrival of other foreign ministers -- which might suggest a deal is at hand -- was not expected this week.

"I don't think their presence will be needed in this round," he told state media.

Iran's nuclear chief Ali Akbar Salehi, also present along with US Energy Secretary Ernest Moniz, however indicated late Wednesday that concrete results could be reached this week.

"We still have two days to go," he told reporters.

The European diplomat said Thursday that Iran was demanding that US Security Council resolutions against it be lifted "on day one" of any deal, but that this was unacceptable.

"There is nothing agreed in any format," he said.

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

Negotiators 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 Benjamin Netanyahu -- whose country is widely assumed to have nukes itself -- 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.