Vienna: Tortuous talks towards a Iran nuclear deal ploughed on Friday with the head of the UN’s atomic watchdog having apparently failed in Tehran to advance a nuclear bomb probe, a major hurdle to the accord.

Iranian President Hassan Rouhani’s chief of staff, Mohammad Nahavandian, meanwhile headed to the negotiations in Vienna, in what the official IRNA news agency called a “special mission”.

“Important progress has been made but questions on technical issues and the wording [of the deal] remain,” Iranian Foreign Minister Mohammad Javad Zarif said on a seventh day of negotiations.

“My impression is that the political will [to get a deal] exists but that this has not yet been transmitted to the bureaucrats” working on the text, Zarif, due to meet US Secretary of State John Kerry later, told Iranian television.

Ahead of a Tuesday deadline, the chief negotiators of Iran, the United States and the European Union haggled for six hours on Thursday night until 3am, a senior US official said.

“We have five days remaining... The technical work is advancing on the main text, on the appendices,” a western diplomat said. “It feels like the end.”

Other foreign ministers besides Zarif and Kerry were expected back in Vienna on Sunday evening and to stay until Tuesday to get the job done, the diplomat said.

The P5+1 — the United States, Russia, China, Britain, France and Germany — want an accord that curbs Iran’s nuclear activities so that making an atomic bomb is all but impossible.

Russia’s top negotiator Sergei Ryabkov on Thursday voiced cautious optimism, saying the document both sides were working on was “91 per cent” finished.

“I can’t predict how many hours it will take to resolve this situation. But all parties are of the opinion that this matter will be resolved in the coming days,” Ryabkov, deputy foreign minister, told Russian news agency TASS.

It will be up to the International Atomic Energy Agency (IAEA) to verify Iran is sticking to its side of the bargain through enhanced inspections of Iran’s nuclear facilities.

But the P5+1 want the watchdog also to be able to visit sites where there is no declared nuclear material to probe alleged efforts, before 2003 and possibly since, to develop a nuclear weapon in secret.

On Thursday the IAEA chief Yukiya Amano visited Tehran to meet Rouhani and others in an attempt to jump-start a stalled probe into these so-called “possible military dimensions” of Iran’s activities.

But after returning a statement suggested that no breakthrough on the issue — which Western powers say is vital for the final deal — had happened.

“I believe that both sides have a better understanding on some ways forward, though more work will be needed,” Amano, who was expected to debrief the later P5+1 on his trip, said in a brief statement.