By Fredrik Dahl and Zahra Hosseinian

VIENNA/ZURICH (Reuters) - The U.N. nuclear watchdog signaled on Friday it would keep trying to secure Iran's cooperation with a long-stalled investigation, but a senior Iranian lawmaker suggested Tehran would only cooperate if it won sanctions relief in return.

The comments by Alaeddin Boroujerdi, who chairs parliament's national security and foreign policy committee, add to Western suspicions that Iran may be using its talks with the U.N. agency as a bargaining chip to win concessions from world powers.

"Lifting sanctions against Iran is a national right of ours ... If we are supposed to have more cooperation with the Agency, Westerners should know that this is a two-way road," parliamentary news agency Icana quoted him as saying.

The U.N. International Atomic Energy Agency has been trying for a year to negotiate a framework agreement with Iran that would enable the Vienna-based IAEA to resume its investigation into suspected nuclear weapons research by the Islamic Republic.

Iran, a major oil producer which is facing tightening Western sanctions, says its nuclear program is entirely peaceful and that Western and Israeli intelligence information pointing to possible weapon aims is forged and baseless.

The IAEA's talks with Iran are separate from, but still closely linked to broader negotiations between Tehran and six major powers aimed at reaching a diplomatic settlement to the decade-old dispute and avert the threat of a Middle East war.

Only the powers can decide to ease sanctions on Iran and their immediate demand is that Tehran curbs its uranium enrichment, work which can have both civilian and military uses, and shuts an underground nuclear facility.

Both Iran and the powers - the United States, Britain, France, Germany, China and Russia - say they want to resume talks after a seven-month hiatus. But they have yet to announce a date and venue for their next meeting.

The IAEA and Iran failed in two days of talks last week to bridge their differences on how the investigation should be conducted and a new session has been scheduled for February 13.

In a meeting with Israel's president this week, IAEA Director General Yukiya Amano underlined the need to resolve differences with Iran diplomatically, the U.N. agency said on Friday, rather than war as Israeli leaders have mooted.

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Israel, widely believed to be the only nuclear-armed state in the Middle East, has threatened possible military action if diplomacy and sanctions fail to prevent Iran from developing nuclear weapons.

Amano told Israeli President Shimon Peres at the World Economic Forum in Switzerland on Thursday that the U.N. agency had intensified "dialogue" with Tehran, the IAEA said.

That was a reference to the IAEA's push since January last year, so far fruitless, to gain access to sites, officials and documents in Iran it says it needs for its inquiry.

Amano "made clear the Agency's commitment to dialogue, and the need to resolve issues with Iran by diplomatic means", the IAEA said in a statement. Continued...