Iran's supreme leader Ayatollah Ali Khamenei on Thursday welcomed comments by US President Barack Obama damping down talk of war against Tehran over its controversial nuclear drive.

"This talk is good talk and shows an exit from illusion," Khamenei's website quoted the leader as telling clerics from the Assembly of Experts, the 86-member body which selects the supreme leader, supervises his activities and can dismiss him.

Obama on Tuesday said that Iran's nuclear programme was not an immediate threat, arguing a "window" for diplomacy could forestall an Iranian bomb, while slamming Republican candidates for their hawkish statements demanding military action against the Islamic republic.

"But the US president continued saying that he wants to make the Iranian people kneel through sanctions, this part of this speech shows the continuation of illusion on this issue," Khamenei added.

Obama stole some of the political limelight from Republican presidential hopefuls by holding his first White House news conference in five months as voters went to the polls in the 10-state primary bonanza dubbed Super Tuesday.

Obama slammed Republicans for "big talk" and "bluster" and failing to consider the costs of war with Iran.

His remarks came a day after Israeli Prime Minister Benjamin Netanyahu said he could not wait "much longer" for diplomacy on Iran to work.

Western governments and Iran's regional arch-rival Israel suspect that Tehran is seeking an atomic weapons capability under the guise of what it insists is a civilian nuclear programme.

The UN Security Council has slapped four sets of sanctions on Iran over its failure to heed repeated ultimatums to suspend uranium enrichment, the most controversial aspect of its nuclear programme.

The European Union and the United States have imposed further, unilateral sanctions aimed at crippling Iran's oil and financial sectors.

"The continuation of this illusion (that sanctions might deter Iran from pursuing its nuclear programme) will hurt the American officials and will lead their calculations to defeat," Khamenei said.

His comments come as long-stalled talks between the major powers and Iran on its nuclear programme are poised to resume.

The so-called P5+1 group of Britain, China, France, Russia and the United States plus Germany called on Iran on Thursday to enter into a "serious dialogue" and "without pre-conditions."

"We call on Iran to enter, without pre-conditions, into a sustained process of serious dialogue, which will produce concrete results," a joint statement said.

They said their readiness to resume negotiations was "on the understanding that these talks will address the international community's longstanding concerns and that there will be serious discussions on concrete confidence building measures."

The P5+1's negotiator, EU foreign affairs chief Catherine Ashton, announced on Tuesday that the powers were ready to resume talks with Iran, but said the date and the venue still had to be agreed.

The last round of talks -- in Istanbul in January 2011 -- broke down over Iran's insistence on discussing "preconditions" before beginning full-blown negotiations on the nuclear dispute, Western diplomats said.

On Wednesday, the speaker of the Iranian parliament, Ali Larijani, warned the new talks would fail too if the major powers resorted to "pressure".

"They (the powers) should pay attention that if they want to continue pressure in the talks, it will achieve nothing," Larijani said.

World powers want 'serious dialogue' with Iran
Vienna (AFP) March 8, 2012 - World powers said Thursday that mooted talks with Iran must be "serious" and urged Tehran to allow UN inspectors access to a military base thought to be central to its suspected nuclear weapons drive.

"We call on Iran to enter, without preconditions, into a sustained process of serious dialogue, which will produce concrete results," said a statement from the United States, China, Russia, France, Britain and Germany, known as the P5+1.

They added that their readiness to negotiate was "on the understanding that these talks will address the international community's long-standing concerns and that there will be serious discussions on concrete confidence building measures."

The statement at a closed-door regular board meeting of the International Atomic Energy Agency, came two days after EU foreign policy chief Catherine Ashton said on behalf of the P5+1 that they were ready for talks with Iran.

Ashton said she did not want to "repeat the experience of Istanbul", referring to the last talks 14 months ago in the Turkish city, which broke down over Iranian demands to discuss "preconditions", according to Western envoys.

No date or location have been agreed but Washington's envoy Robert Wood to the IAEA told reporters Thursday he expected the talks to happen "in the coming few months."

Iran's envoy to the IAEA, Ali Asghar Soltanieh, declined to be drawn meanwhile on what concessions Iran might be prepared to make in the upcoming talks.

Tehran's willingness to discuss with the IAEA allegations of a nuclear weapons programme "by itself is confidence building," he told reporters.

"We will never suspend our nuclear activities, and we will continue (with them), under the supervision of course of the IAEA," he said.

The possible resumption comes despite an apparent deadlock between the IAEA and Iran after two fruitless visits to Tehran led by chief inspector Herman Nackaerts in January and February.

The visits saw Iran again reject a major IAEA report in November outlining a host of suspicious nuclear activities and deny access to the Parchin military site where explosives testing for warhead research allegedly took place.

Thursday's P5+1 statement urged Iran to "fulfil its undertaking to grant access to Parchin."

Amano on Monday appeared to allege that Iran was removing evidence at the base, saying "activities" spotted by satellite "makes us believe that going there sooner is better than later."

Iran says the IAEA already visited Parchin near Tehran in 2005 and that it is under no obligation to allow access because the site is non-nuclear.

Nackaerts said that since 2005 "we have acquired new information -- from satellite imagery -- from which we have been able to identify the precise location where we believe an explosive chamber is situated."

Iran is highly sensitive about allowing access to military sites following a large explosion at the Bid Ganeh base in November and multiple assassinations of nuclear scientists it has blamed on Israel and the United States.

"The agency heard a lot of excuses from Iran," Wood, the US envoy, said Thursday. "When you have these reports about a possible clean-up going on, and the fact that the agency has been denied access, makes you wonder."

Soltanieh said allegations of "sanitization" of the site were "a childish (and) ridiculous story made out of nothing," describing Parchin as "one minor issue" in its negotiations with the IAEA, which he said were ongoing.

"Unfortunately Western countries are not telling the truth ... The truth is that Iran is not pursuing nuclear weapons," with sanctions having "no effect whatsoever" on Iran's nuclear activities, including enrichment, he said.