(Adds background, details)

By Peter Graff

BAGHDAD, Aug 6 (Reuters) - U.S., Iranian and Iraqi officials held the first meeting in Baghdad on Monday of a sub-committee intended to improve cooperation on Iraqi security among the three countries, officials said.

"It's the sub-committee they've been talking about for some time now. They're meeting today at the expert level. It's hosted and organised by the Iraqis," U.S. embassy spokesman Philip Reeker said.

The U.S. delegation was headed by Marcie Ries, minister-councillor for political-military affairs at the U.S. embassy, Reeker said.

Iranian Foreign Ministry spokesman Mohammad Ali Hosseini said Tehran's delegation was being led by the deputy head of its mission, Amir Abdollahian.

Setting up the security sub-committee was one of the main achievements of a July 24 meeting in Baghdad between the U.S. and Iranian ambassadors in Iraq.

Washington accuses Tehran of fomenting instability in Iraq, supporting militias and providing weapons, such as armour-penetrating bombs, used to kill U.S. troops.

Tehran denies the charge and blames Iraq's unrelenting sectarian violence on the 2003 U.S.-led invasion to topple Saddam Hussein.

The talks between the United States and Iran, which have not had diplomatic relations for almost 30 years, had their first round in May and are seen as groundbreaking.

U.S. ambassador Ryan Crocker said after the July 24 meeting that he had told his Iranian counterpart, Hassan Kazemi-Qomi, that Washington believed attacks by militias associated with Iran had increased over the previous two months.

He said those talks, which lasted seven hours, had involved "heated exchanges". Iraqi Foreign Minister Hoshiyar Zebari called them "very challenging". (Additional reporting by Parisa Hafezi in Tehran)

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