The head of the UN nuclear watchdog will head to Iran this weekend to pin down an Iranian pledge, made at talks with big powers on Thursday, to open a newly revealed uranium enrichment site to inspections.

The Geneva meeting, which also yielded agreement on follow-up talks before the end of October, lowered tensions a notch in a protracted standoff over suspicions that Iran is secretly trying to develop nuclear weapons.

But Western officials said Iran should give access to the enrichment site quickly -- within two weeks, some said -- and go farther in gestures of transparency at the next talks to gain a longer respite from the threat of tougher UN sanctions.

Iran emerged from the talks looking more cooperative but avoided the main issue by insisting on a sovereign right to atomic energy, again sidestepping an offer of trade incentives for suspending nuclear activity that has potential military applications.

A European diplomatic source said Iran would be pushed at the next meeting to address a demand it freeze any expansion of enrichment capacity, as an interim step toward suspension.

Iran also gave no ground on UN demands for unfettered UN inspections to verify that it is not hiding other nuclear production or research sites that would raise concerns about clandestine military intentions, as Western officials suspect.

"Yesterday's talks were clearly a first step but others (by Iran) must follow," German government spokesman Andreas Peschke told a news conference in Berlin.

US President Barack Obama spoke of a "constructive beginning", but said Iran must do much more to prove it was not accumulating enriched uranium for the purpose of producing atom bombs, rather than just fuel for atomic power plants as it says.

Russian Foreign Minister Sergei Lavrov said the tentative deals reached in Geneva "inspire cautious optimism" and added that it was important "to make sure these agreements are fully and timely met," Interfax news agency reported.

RIA news agency quoted Lavrov as saying Iran agreed to grant International Atomic Energy Agency inspectors access to its second enrichment site "to resolve all the issues around (it)."         

Dates and conditions

Diplomats said IAEA chief Mohamed ElBaradei would fly to Tehran on Saturday and stay throughout Sunday to flesh out dates and conditions for IAEA access to the site, buried deep inside a mountainside near the Shi'ite holy city of Qom.

ElBaradei said this week that Iran was "on the wrong side of the law" in failing to declare the plant as soon as plans were drawn up. US, British and French intelligence services have determined that construction began 3-1/2 years ago.

Iran, which reported the remote site to the IAEA on September 21, said the time when UN monitors would be able to go there was "not far away", but offered no time frame.

The European diplomatic source said Iran "did not say no" when Iran was prodded in Geneva to permit an IAEA visit to the site within two weeks. "It (timetable for access) will be a way to gauge if they are being serious," he said.

"This (is) a matter of some urgency ... not only just to open it up but also make sure ... the IAEA would be able to talk to some of the engineers there and see documents and plans," US State Department spokesman Ian Kelly said in Washington.

ElBaradei was last in Iran in January 2008 to nudge Iran into implementing steps, still incomplete, to clarify concerns about possible military dimensions to its nuclear programme.

Western officials said Iran had agreed "in principle" to ship out most of its low-enriched uranium for reprocessing in Russia and France. It would then be returned to power a Tehran reactor that makes medical isotopes for cancer care.

Western officials say this would reduce an enriched uranium stockpile in Iran that coul