"We have agreed to launch talks in Istanbul on April 14," Michael Mann said on Monday. "We hope that this first round will produce a conducive environment for concrete progress.

"We are of course aiming at a sustained process," Mann added.

Iran last held talks with the so-called P5+1 powers -- permanent UN Security Council members Britain, China, France, Russia and the United States plus Germany -- in January 2011 with no results.

There had been a dispute as to whether the venue for the fresh talks should be Istanbul or Baghdad.

US Secretary of State Hillary Clinton had already announced that the talks between Tehran and the group of six powers would resume April 13-14 in Istanbul.

However Tehran had then objected to the venue, arguing that Turkey's tough line against Damascus in recent months did not make it a neutral host, and suggested Baghdad instead.

Iran did not specifically react to the dispute over the venue Monday but its foreign minister said he hoped all sides would come to the negotiating table without pre-conditions and make the fresh talks succeed.

"We hope the P5+1 will come to the negotiating table with honesty, and we also will make an honest effort so that both sides reach a win-win conclusion," Ali Akbar Salehi told the Iranian parliament's website.

Both sides have sent positive signals ahead of the fresk talks, with Iran hinting it had a new offer to put on the table and reports that Washington could accept a civilian nuclear programme if Tehran could prove it was not seeking the atomic weapon.

Egged on by Israel, its chief ally in the Middle East, Washington had ratcheted up the rhetoric against the Islamic state in recent months, sparking fears of a military flare-up.

Observers hope the fresh round of talks between Iran and the five permanent members of the UN Security Council and Germany will help defuse the risk of further escalation.

Oil prices slid in Asian trade on Monday with New York's main contract, West Texas Intermediate crude for delivery in May, shedding $1.11 to $102.20 per barrel.

Yukio Hatoyama, whose short stint in the top job ended in June 2010 after just nine months, was publicly admonished by his own party after reportedly criticising the UN's nuclear watchdog for "double standards".

During a trip in which he met with Iranian President Mahmoud Ahmadinejad, Hatoyama said Tehran was not being treated properly by the International Atomic Energy Agency.

"International trust-building and respecting regulations are important issues for the world community," he said, according to a statement issued by Ahmadinejad's office.

"They should be seriously pursued given the double standards by the IAEA towards some nations, including Iran, which is not fair."

Hatoyama late Monday denied making the comments and said Tehran had "completely fabricated" them, Kyodo news agency reported.

"I have made no comments that deviate from the stance of the Japanese government," Kyodo quoted him as saying on his return to Tokyo.

Hatoyama's reported comments in Iran had come under fire from Tokyo, which said he was at odds with the official position.

"Japan respects the IAEA's role in solving nuclear-related issues," Chief Cabinet Secretary Osamu Fujimura told reporters.

"Japan is asking Iran to thoroughly cooperate with the IAEA so that it can solve pending issues over its nuclear programme."

Hatoyama, who has emerged as something of a loose cannon since being forced from office, was already under a cloud for the trip after Foreign Minister Koichiro Gemba expressed concerns that it could undermine international action against Tehran.

"The (Japanese) government is taking a consistent position that it would be better if he had not gone (to Iran) at a time like this, even if it is a personal trip," Fujimura said.

The West, led by the US, believes Iran is developing nuclear weapons and is pressing ever-tighter sanctions against the regime. Tehran insists its atomic programme is peaceful and purely for energy.

Ahmadinejad told Hatoyama on Sunday that Iran opposes nuclear weapons, his official website reported.

"The Islamic Republic of Iran is fundamentally opposed to the atomic bomb and weapons of mass destruction," Ahmadinejad told Hatoyama.

"Iran and Japan can exert a common effort to create a world without atomic weapons... Difficult but humanitarian efforts will win in the end."

Talks between Iran and the five permanent UN Security Council members and Germany are due to start Saturday in Istanbul.

Hatoyama, the millionaire scion of an influential family, swept his Democratic Party of Japan (DPJ) to a stunning election victory in September 2009, ending more than half a century of almost unbroken conservative rule.

But his short-lived premiership was blighted by a reputation for crippling indecision.

Despite his status as a senior adviser to the DPJ, he has struggled to find a role in public life since joining the ever-swelling ranks of former Japanese prime ministers.