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The Iraqi government has dispatched a delegation to Iran to discuss concerns about the arming and training of Shiite militias in Iraq, an official said Thursday.

Five Shiite politicians left Wednesday with "evidence, confessions and pictures" indicating that Iran is supplying weapons and training fighters who are locked in a violent standoff with U.S. and Iraqi troops, the government official said.

The official, who spoke on condition of anonymity because he wasn't authorized to release the information, said the delegation would present the Iranian government with material that proves Iraqi gunmen are being armed and trained by Iran's elite Quds Force.

The delegation was expected to meet with Iran's supreme leader Ayatollah Ali Khamenei to discuss the issue and stress that ongoing clashes between Shiite extremists and U.S.-Iraqi forces were threatening political gains by Shiites.

The U.S. military has stepped up its allegations that Iran is continuing to train and arm Shiite militias despite Tehran's insistence that it is not fueling violence in Iraq but trying to promote stability in the neighboring country.

On Wednesday, two senior Pentagon officials said Iraqi leaders have been given the latest U.S. evidence of Iranian support for militias inside Iraq.

U.S. military officials have said that the evidence includes caches of weapons that have date stamps showing they were produced in Iran this year.

The weapons include mortars, rockets, small arms and armor-piercing roadside bombs known as explosively formed penetrators, or EFPs, that troops have discovered in recent months, according to another senior U.S. military official.

The Iraqi leaders are hoping to pressure Iran to stop aiding militias by presenting Tehran with the latest evidence, a senior U.S. defense official said Wednesday.

Reda Jawad Taqi, a Shiite lawmaker and a senior member of the Supreme Islamic Iraqi Council, confirmed the delegation left for Iran on Wednesday.

Taqi did not provide details but said the purpose of the trip was "to review the current situation in Iraq with Iranian officials, to clarify the situation with them and to enhance the mutual relationship between the two countries."

U.S. Defense Secretary Robert Gates and other U.S. government officials claim that Iranian-trained Shiite militiamen are carrying out attacks in Baghdad and elsewhere in Iraq — using weapons supplied by Iran — that are killing not only Iraqis but U.S. troops as well.

Clashes between the U.S.-backed Iraqi forces and the militias have escalated since anti-American cleric Muqtada al-Sadr rejected the government's conditions to surrender arms and militia and threatened to unleash an "open war" against the U.S. led forces.

Prime Minister Nouri al-Maliki has threatened to politically isolate al-Sadr if he does not disband his powerful Mahdi Army.

Al-Sadr is believed to be in Iran, but the Iraqi delegation has no plans to meet him, the Iraqi official said.