Weary negotiators hoped to see the light at the end of the tunnel Thursday after talking until dawn, but cautioned they were still haggling over the last details to clinch a ground-breaking Iran nuclear deal.

US Secretary of State John Kerry and his Iranian counterpart Mohammad Javad Zarif talked through the night, going line by line over their differences in a bid to seal the outlines of a framework accord to cut back Iran's nuclear ambitions, diplomats close to the talks said.

They made "significant progress," but there is no "final result yet," Zarif told reporters at the luxury lakeside Lausanne hotel hosting the marathon negotiations for the past eight days.

The six powers "have to examine among themselves the results of the negotiations. We don't know yet the result of those discussions."

But a Western diplomat cautioned "the conclusion is far from being imminent."

And German Foreign Minister Frank-Walter Steinmeier cancelled a planned trip to the Baltics to stay at the talks in Switzerland.

In a sign that eight days of negotiations may be drawing to a close, Zarif said he and EU foreign policy chief Federica Mogherini would make a "joint declaration" to the press if everyone was in agreement.

"But the text still has to be worked on," he said.

After 18 months of intense negotiations, the six world powers and Iran are hoping to pin down the main contours of a deal to put a nuclear bomb out of Iran's reach.

The aim is to turn this into a comprehensive accord backed by specific technical commitments by June 30 when an interim deal struck in November 2013 expires.

After just a few hours sleep, the six world powers -- Britain, China, France, Germany, Russia and the United States -- were back at the negotiating table.

The stakes were very high, French Foreign Minister Laurent Fabius said, adding that at issue was the question of non-proliferation, and "Iran's reintegration into the international community."

Under the interim 2013 accord, which has been extended twice and which expires on June 30, Iran froze certain nuclear activities in return for minor sanctions relief.

Success in Lausanne would end a 12-year-old standoff. Failure may set the United States and Israel on a road to military action to thwart Iran's nuclear drive.

The White House said the talks were still "productive" and progress was being made.

"But if we are in a situation where we sense that the talks have stalled then yes, the United States and the international community is prepared to walk away," said White House spokesman Josh Earnest.

Iran's chief negotiator Abbas Araghchi had said there were two main sticking points -- a mechanism for lifting crippling sanctions against the Islamic republic, and the country's research and development of new nuclear machinery.

Global powers want Iran to scale down its nuclear programme to extend the "breakout" time needed to assemble enough nuclear material to make a bomb, which Iran has always denied seeking.

But Iranian negotiators are under pressure from domestic hardliners not to give too much away -- while also delivering on President Hassan Rouhani's promise to win the lifting of sanctions.

Global powers have refused an immediate end to all sanctions, preferring instead a phased suspension to enable them to be put back in place if Iran violates the deal.

The issue of suspending UN sanctions is particularly tricky -- Iran is also subject to US and EU ones -- with discord among the powers about the mechanism for a "snap-back" mechanism if needed.

US President Barack Obama also needs a deal which he can sell to hostile Republicans in Congress, who remain sceptical of Iran's pledges and are threatening to push for new sanctions from April 14.

Republicans and US allies like Israel and Saudi Arabia fear that if too much of Iran's nuclear programme is left intact, it will still have the ability to obtain a nuclear bomb.