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 3:00 am (0100 GMT), 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 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.
In exchange Iran, which says its programme is for peaceful purposes like electricity generation and not to get the bomb, would see painful sanctions progressively lifted.
It would end a 13-year standoff over Iran`s suspect nuclear programme, and draw the curtain on almost two years of intense negotiations since Rouhani came to power in August 2013.
Russia`s top negotiator Sergei Ryabkov on Thursday voiced cautious optimism, saying the document both sides were working on was "91 percent" 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 jumpstart 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.
Iran rejects the allegations, saying they are based on bogus intelligence provided to a gullible and partial IAEA by the likes of the CIA and Israeli intelligence agency Mossad.
Abbas Araghchi, Iran`s lead negotiator in Vienna, told Iranian media Friday that Tehran was "ready to cooperate with Mr Amano so that it can be proved that these accusations and claims.. are baseless".
Apart from the PMD issue, other difficult topics include the timing and pace of sanctions relief and Iran`s future research and development into newer kinds of nuclear equipment.