The International Atomic Energy Agency won modest gestures from Iran in a first round of talks in Tehran on July 13, like permission for IAEA inspectors to revisit a heavy-water reactor under construction and a pledge to produce a plan for better IAEA access to Iran's underground uranium enrichment plant. Ali Asghar Soltanieh, Iran's ambassador to the IAEA, said its deputy nuclear negotiator Javad Vaeedi would reconvene with the agency's nuclear safeguards director Olli Heinonen at IAEA headquarters in Vienna on Tuesday morning.

"The second round will be held, continuing the discussions of the modalities on how to deal with the outstanding issues, at the same level as in Tehran, and will go on all day," said Soltanieh, who will take part. "The agenda is clear, a work plan to deal with these issues. I don't know (if there will be a breakthrough)."

The IAEA declined immediate comment.

European diplomats told Reuters last week that Western powers had quietly shelved efforts to toughen UN sanctions against Iran until September to see whether Tehran stopped obstructing IAEA investigations ongoing since 2003.

IAEA Director Mohamed Elbaradei has said Iran's pledge last month to work out an action plan within 60 days to counter suspicions it is secretly trying to build atomic bombs has raised hope of defusing a volatile standoff between Tehran and Western powers.

ElBaradei has also cited what he said was a slowdown in the expansion of Iran's uranium enrichment detected by IAEA inspectors during a visit to the Natanz plant in early July.

Tehran has threatened to call off its rapprochement with UN investigators if the West moves to pass another sanctions resolution on top of two others enacted since December.

Iran says it is refining uranium only to generate more electricity and allow it to export more of its bountiful oil.

Among the issues the IAEA wants to get to the bottom of are the origin of traces of highly enriched -- or weapons-capable -- uranium found on some equipment, experiments with plutonium, and the status of Iranian research into advanced centrifuges that can enrich three times as fast as the model Iran now uses.

The talks have also addressed steps Iran could take towards more transparency about its declared nuclear activity.

Iran approved an inspector trip to the Arak heavy-water complex, a possible future source of bomb-grade plutonium in Western eyes, before the end of July, four months after halting such access in protest at existing sanctions.

Iran also agreed to accredit five of 10 inspectors the IAEA proposed to help replace 38 from Western states barred early this year, and to finalise arrangements in early August for closer IAEA monitoring of the Natanz plant, diplomats said.