A prominent Iranian lawmaker said on Sunday Iran would never agree to shut down its Fordo underground nuclear enrichment facility as demanded by world powers, Mehr news agency reported.

"It is possible that they set some conditions such as shutting down Fordo, which definitely will not happen," Mehr quoted Alaeddin Boroujerdi, head of the parliament's foreign policy committee, as saying.

Fordo, with nearly 3,000 centrifuges and dug deep into a mountain near the holy city of Qom, some 150 kilometres (90 miles) south of Tehran, is at the heart of international concerns over Iran's nuclear drive.

The site, whose existence was revealed in 2009, began in late 2011 to enrich uranium to purities of 20 percent, a few technical steps away from the 90-percent level needed for a nuclear weapon.

Iran says it is enriching to this level to provide fuel for the Tehran Research Reactor, which produces medical isotopes, and denies seeking or ever having sought nuclear weapons.

Closing Fordo or limiting enrichment activities has been a key demand by six world powers -- permanent UN Security Council members Britain, China, France, Russia and the United States plus Germany -- in negotiations with Iran over its controversial nuclear programme.

In return, the powers are offering to ease some sanctions against the Islamic republic, such as those imposed on trade in gold and on the petrochemical sector.

Iran and the P5+1 resumed talks mid-October in Geneva during which Tehran presented a new proposal that its chief nuclear negotiator Abbass Araqchi said could settle the dispute "within a year".

Experts from both sides are to meet at the end of this week in Vienna to prepare for the next round of talks, in Geneva on November 7-8.

Israel and to a lesser degree the US have refused to rule out military action against Iran should it continue its nuclear enrichment programme.

However, Boroujerdi warned against any military action, saying Iran was prepared to deter any foreign attack.

"We have created the conditions for America and the Zionist regime in a way that they will never think of attacking our nuclear sites. Our missiles are a deterrent ... but Fordo is one of our red lines," he said, without elaborating.

Some experts warn that Iran next year may reach "critical capacity" -- the point at which it could, in theory, process enough weapons-grade uranium for a bomb before being detected.

But since becoming president in August, Hassan Rouhani, seen as a relative moderate, has raised hopes that the long-running crisis can be resolved and threats of military action silenced for good.