* Six powers to meet on Iran's nuclear program Sept. 2

* Merkel says Iran's energy sector may be sanctioned

* Russia, China reluctant to back more Iran sanctions (Adds Russian statement on Sept. 2 meeting)

By Louis Charbonneau

UNITED NATIONS, Aug 20 (Reuters) - Western powers, which are considering new sanctions against Iran, are urging the U.N. nuclear watchdog to reveal all the information it has that suggests Tehran is pursuing atomic weapons, diplomats said.

The United States, Britain, France and Germany are planning to hold high-level talks on Iran's nuclear program with Russia and China on Sept. 2, Russia's Foreign Ministry said.

Western diplomats said there would probably be another meeting on Iran in New York on the sidelines of the U.N. General Assembly later in the month.

At those meetings, diplomats say, the four Western powers will attempt to convince Moscow and Beijing that a fourth round of U.N. sanctions target Iran's energy sector may be necessary, an idea Russia and China have resisted. Both Moscow and Beijing have strong trade relations with the Islamic Republic.

The six powers have offered Iran economic and political incentives in exchange for freezing its uranium enrichment program, but Tehran has not responded to the offer and has refused to halt any of its nuclear activities. It insists its atomic program will only produce electricity, never weapons.

German Chancellor Angela Merkel said in an interview with a German newspaper that if no progress is made in talks with Iran, it might have to face "further sanctions" targeting its energy sector. Three rounds of U.N. asset freezes and travel bans focused only on Tehran's nuclear and missile industries.

U.S. President Barack Obama has given Iran until September to take up the six powers' offer or face harsher penalties. The Russian Foreign Ministry statement said that Russian Foreign Minister Sergei Lavrov and U.S. Secretary of State Hillary Clinton discussed the Sept. 2 meeting on Iran on Thursday.

To make a strong case for more sanctions to Russia and China, diplomats said the four Western powers have been urging Mohamed ElBaradei, the outgoing head of the Vienna-based U.N. nuclear watchdog, to reveal information on Iran he has and which they say could help bring Russia and China around.

'ALLEGED' NUCLEAR WEAPONS STUDIES

ElBaradei is expected to circulate his latest International Atomic Energy Agency report on Iran to the agency's governing board and the U.N. Security Council as early as next week. Western diplomats said they hoped he would include unreleased information the IAEA has gathered on Iran.

One senior Western diplomat said some of that information related to Iran's so-called "alleged studies" into building an actual nuclear weapon.

"The alleged studies are more than alleged," the diplomat said on condition of anonymity, adding that ElBaradei has for years been "overly cautious" about his Iran reports.

Romain Nadal, a spokesman for the French Foreign Ministry, told reporters in Paris on Wednesday that France would like the new IAEA report on Iran to be as comprehensive as possible.

"Several pending questions posed by the IAEA, relating to activities with a military dimension, still remain unanswered," Nadal said. "We want the next IAEA report to be as exhaustive as possible, including on this (military) aspect."

The diplomats acknowledged that the IAEA does not have any hard evidence proving undeniably that Iran wants atomic bombs.

However, several said the agency does have credible information suggesting that a U.S. intelligence estimate that Iran ended an atomic weapons program in 2003 was incorrect.

"It would be useful if all the evidence and the agency's assessment be included in a summary form in an annex to be attached to the next regular IAEA report on Iran," a Western diplomat said. "This would also help counter some of the false or misleading arguments put forward by Iran."

Some Western and Israeli officials have gone further over the years -- accusing ElBaradei of deliberately understating the case against Iran to undermine the U.N. sanctions drive.

A Vienna diplomat close to the IAEA angrily dismissed suggestions that the agency has been withholding evidence.

"It is deeply disturbing that baseless and malicious allegations have been made," the diplomat said, adding that the agency relied on an "independent, impartial and exhaustive verification and investigation procedure." (Additional reporting by Sylvia Westall in Vienna, Noah Barkin and Madeline Chambers in Berlin; editing by Cynthia Osterman and Mohammad Zargham)

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