Nuclear talks towards a lasting agreement between Iran and six powers appeared to make modest progress on a second day Wednesday, with Washington saying the negotiations were "constructive and useful".

Iranian state media said both sides were close to agreeing a framework agreement on how negotiations would proceed in future rounds over the coming months in what promises to be a lengthy process.

US State Department spokeswoman Marie Harf said that the Vienna talks between Iran and the United States, Russia, China, Britain, France and Germany had been "constructive and useful".

Negotiators discussed "both process and substance," she said.

Speaking to reporters, Harf declined to comment further, saying only that the talks would continue for a third day in the Austrian capital on Thursday.

Iranian Foreign Minister Mohammad Javad Zarif and EU foreign policy chief Catherine Ashton, the powers' chief negotiator, were expected to hold a closing news conference on Thursday morning.

The parties hope to create a lasting accord out of an landmark interim deal struck in November, under which Iran agreed to freeze certain nuclear activities for six months.

In exchange, the Western governments offered minor relief from a range of punishing sanctions that have cost Iran billions of dollars in lost oil revenues, as well as a promise of no new sanctions.

The six-month deal expires on July 20 but can be extended, with the parties aiming to conclude negotiations and implement the final "comprehensive" deal by November.

- Shared objective -

Zarif had said late Tuesday that the talks had "started on the right track".

"We have a shared objective, and that is for Iran to have a nuclear programme that is exclusively peaceful," he said.

He said a deal was "totally achievable" but would take more than "one or two sittings" and would require "some innovation and some forward thinking".

Others have been considerably more circumspect about the prospects for a deal that satisfies hardliners on both sides, as well as other countries such as Israel and Sunni monarchies in the Gulf.

Iran's supreme leader Ayatollah Ali Khamenei, who retains the final say on all matters, said on Monday that this effort would "go nowhere" but that he was not against trying.

The aim of the final deal would be for Iran to retain its civilian nuclear programme, but likely on a reduced scale and with enhanced oversight to ensure a dash for nuclear weapons is all but impossible.

In exchange for a full lifting of sanctions, the powers want Iran's nuclear programme to be within what the Geneva deal called "mutually agreed parameters consistent with (Iran's) practical needs" and for a "long-term duration".

Speaking in Iran, meanwhile, the commander of the powerful Revolutionary Guards, General Mohammad Ali Jafari, warned on Wednesday against crossing "red lines".

Tehran has previously indicated its opposition to any dismantling of nuclear facilities, even though the chief US negotiator, Wendy Sherman, has said Iran "does not need" the Fordo site or a new heavy-water reactor at Arak.

Jafari said that Iran "will be victorious either way" in the talks.

Iran has long been suspected of seeking atomic weapons, despite its denials, and the US and Israel -- the latter assumed to have a large atomic arsenal itself -- have never ruled out military action.

Further upping the ante between the two foes, Iran's foreign ministry on Wednesday blamed a double suicide car bombing near an Iranian cultural centre in Beirut that killed six people on Israeli "agents".

The attack was quickly claimed by the Abdullah Azzam Brigades, a jihadist group inspired by Al-Qaeda that previously claimed a double suicide bombing aimed at Iran's embassy in Beirut.