The White House has said progress is still being made in crunch nuclear talks with Iran, but warned the US would walk away from the discussions in Switzerland if negotiations stall.

The talks between Iran and six world powers in the city of Lausanne have now been extended into Thursday, despite what had originally been a Tuesday midnight deadline, in an effort to hammer out the outline of an agreement on curbing Tehran's nuclear programme.

"The sense that we have is that, yes, the talks continue to be productive and that progress is being made," said White House spokesman Josh Earnest.

"As long as we are in a position of convening serious talks that are making progress," Earnest said, Washington "would not arbitrarily or abruptly end them."

"But if we are in a situation where we sense that the talks have stalled then yes, the US and the international community is prepared to walk away."

He added that US President Barack Obama would make a public address when negotiations conclude, whether successful or not.

Kerry stays

US Secretary of State John Kerry said late on Wednesday that he will stay in Lausanne until at least Thursday morning to continue the negotiations, his spokeswoman Marie Harf said.

For weeks, Kerry and his Iranian counterpart, Javad Zarif, have been meeting in an intense effort to reach a political understanding on the terms for a deal.

The US, Russia, China, Britain, France and Germany want Iran to scale down its nuclear programme to extend the "breakout" time needed for it to assemble enough nuclear material to make a bomb.

Meanwhile, Iran's foreign minister said that the nuclear talks could succeed if all those taking part have the political will to resolve Iran's 12-year old nuclear standoff.

"We are trying. We are doing our best to be able to move forward and be able to resolve this issue and I hope we can all make progress,” Zarif told Al Jazeera's diplomatic editor James Bays.

"It [a deal] depends on all of these seven countries that need to move forward. I for one have been ready and am ready with my delegation to move forward tonight. Or tomorrow or whenever is appropriate."

Zarif also said that his delegation was ready to stay in Lausanne as long as is useful and necessary.

British Foreign Secretary Philip Hammond sounded a note of caution. "I think we have a broad framework of understanding, but there are still some key issues that have to be worked through," he told the BBC.

German Chancellor Angela Merkel said she hoped a compromise would be reached later on Wednesday.

"I hope and I wish that a compromise will be reached today that corresponds to the conditions we have set - namely that Iran gets no access to nuclear weapons," Merkel said at a news conference with the president of Kyrgyzstan.

Key issues remain

"Talks are still ongoing. It seems still possible to reach an agreement, but the question is if it is going to be a framework deal or a comprehensive one," Bays said.

French Foreign Minister Laurent Fabius, along with China's Wang Yi, left the talks early on Wednesday.

Fabius returned later in the day, saying the marathon negotiations were in the final but most difficult stretch.

"We are a few metres ... from the finishing line, but we are well aware that the final metres are the hardest," Fabius told reporters, saying "it is not over yet."

A historic deal is being held up by arguments over the mechanism for lifting crippling sanctions against the Islamic Republic.

Obstacles remain on several main issues - uranium enrichment, where stockpiles of enriched uranium should be stored, limits on Iran's nuclear research and development, and the timing and scope of sanctions among other issues, according to negotiators.