VIENNA: Iran has offered "constructive solutions" to resolve disputes in nuclear talks with six major powers, the Iranian Students News Agency (ISNA) reported on Wednesday, but Western officials suggested they had heard nothing new from Tehran.

Iran and the powers are in the last stretch of talks to reach a final agreement to end a more than 12-year standoff over Iran's nuclear programme. The aim is to lift sanctions in exchange for at least a decade of curbs on the programme.

"Iran has presented constructive solutions to overcome the remaining differences. We will not show flexibility regarding our red lines," an Iranian diplomat, who was not identified, told ISNA.

But Western officials indicated they had yet to see substantive new proposals. The biggest sticking points include issues such as a United Nations arms embargo, U.N. missile sanctions, the speed of sanctions relief, and research and development on advanced nuclear centrifuges.

"I haven't seen anything new from Iran," a Western diplomat close to the talks told Reuters on condition of anonymity. Another Western official echoed the remarks.

US Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif have stayed behind in Vienna, along with EU foreign policy chief Federica Mogherini, in an attempt to break the logjam while most of the other foreign ministers returned to their capitals.

Air-conditioning systems in the luxurious Palais Coburg hotel are struggling with outside temperatures approaching 40 Celsius (104 Fahrenheit), and some negotiators have found it hard to keep their cool during the discussions, officials say.

TENSE EXCHANGE

Kerry and Zarif were involved in a tense exchange over UN sanctions on Monday night, diplomats said. Tehran says conventional weapons and missiles have nothing to do with the nuclear issue and embargoes should therefore be removed.

"There was no slamming of doors but it was a very heated exchange of views," one of the senior Western diplomats said.

Iran's official news agency IRNA quoted unnamed residents at the Palais as saying Kerry and Zarif could be heard shouting at each other during a one-on-one meeting on Monday. A Kerry aide had gone in to tell them that they could be heard clearly.

Western countries accuse Iran of seeking the capability to build nuclear weapons, while Tehran says its programme is peaceful.

A successful deal could be the biggest milestone in decades towards easing hostility between Iran and the United States, foes since Iranian revolutionaries stormed the U.S. embassy in Tehran in 1979.

It would also be a political success for both US President Barack Obama and Iran's pragmatic President Hassan Rouhani, both of whom face scepticism from powerful hardliners at home.

Iran, the United States, Britain, France, Germany, Russia and China have given themselves at least until Friday, but a source from one of the powers said on Tuesday they had to wrap up in the next 48 hours.

A senior US official told reporters on Tuesday: "I believe we will in the near term either get this deal or find out we can't."

The disagreements over UN Security Council sanctions are among the most difficult.

Russia and China, which have never hidden their dislike of sanctions, had indicated they would support the termination of the United Nations arms embargo and missile sanctions on Iran, both of which date back to 2006.

MIXED MESSAGE

In the end, however, they agreed not to break ranks with the Americans and Europeans, who want to maintain the measures given the instability in the Middle East.

"In the current context, it would be pretty obscene as a political message if we resolve the nuclear issue but then give them money and the capacity to import and export arms," a senior Western official said.

Russia is especially sensitive about sanctions, Western officials say, due to the fact that it itself is under US and European Union sanctions over allegations that it is supporting pro-Moscow rebels in eastern Ukraine, which it denies.

French Foreign Minister Laurent Fabius and British Foreign Secretary Philip Hammond were expected to return to Vienna on Wednesday evening.

US and European officials have indicated that they are prepared to walk away if there is not a deal soon, while the Iranians have said they are happy to continue negotiating.

Iran's Supreme Leader Ayatollah Ali Khamenei, who has the power to block a deal, last month ruled out either a long freeze of sensitive nuclear work or opening military sites to inspectors. Western officials say Khamenei's "red lines" have made things more difficult for the Iranian delegation.

"There is a sort of 'good-cop/bad-cop' between Zarif and the supreme leader," a Western official said. "Zarif is under a lot of pressure."

The latest extension of the talks to Friday left open the possibility an agreement would not arrive in time to secure a 30-day review period by the Republican-dominated US Congress.

If a deal is sent to Congress after July 9, the period grows to 60 days, increase the chance that the deal could unravel.