(CNN) -- American officials say there is strong evidence that Shiite militias in Iraq are using Iranian weapons to attack U.S. troops, comments that followed reports that Iraqi security forces began cracking down on the groups near Iran's border.
Materials found following recent attacks against U.S. troops "have been traced back to Iranian origin," an official told CNN. The claim supports the U.S. military position that the militias are backed by Iran, a charge Tehran has long denied.
The material collected indicates the militias are using Iranian materials in "Improvised Rocket-Assisted Munitions" attacks against U.S. forces, the official, who spoke on condition of anonymity, said Saturday. The official was not authorized to release the information to the media.
The munitions, or IRAMS, are metal canisters packed with explosives and propelled by rockets. IRAMS along with armor-piercing grenades are believed responsible for the majority of U.S. deaths in recent months, the official said.
The official acknowledged the weapons could have been assembled and manufactured elsewhere, but said IRAMS are a signature weapon of the militant group Kataib Hezbollah, which is "funded by Iran and receives training there. "
The news about the weapons arose as Iraqi security forces began a crackdown in the volatile Maysan province, a Shiite stronghold in southern Iraq that serves as one of the main arteries from Iran.
The U.S. military has not been asked by the Iraqi government "to provide any substantial assistance so far to the operations reported in Maysan province," Army Maj. Gen. Jeffrey Buchanan, the top U.S. military spokesman in Iraq, told CNN.
Iraq's southern provinces make up the majority of the Shiite heartland, home to the holy cities of Karbala and Najaf. It is in this area where a majority of the Shiite militia attacks have been launched against U.S. troops, who operate from a number of bases in the region.
"While it is clear these militias receive training and weapons from Iran -- using them as proxies for Iran's own agendas in the region -- we have no evidence of Iranian forces being directly engaged in the recent violence in Iraq," Buchanan said in an email late Saturday.
Buchanan said the United States is encouraged by the Iraqi operation, though the security in Iraq remains complex.
While violence has dramatically fallen off since the height of sectarian violence that nearly tore the country apart, attacks and bombings remain a daily occurrence.
The recent increase in attacks pushed June's death toll to the highest this year for Iraqi security forces and civilians. More than 270 people, including 155 civilians, were killed in attacks, Iraqi authorities said.
Fourteen U.S. soldiers were also killed in combat-related incidents in June, the largest loss of life among U.S. troops in Iraq since 2008, according to CNN figures.
The roughly 47,000 American troops in Iraq are due to withdraw by January 1, 2012, under a U.S.-Iraqi security pact, though Iraqi Prime Minister Nuri al-Maliki is consulting with lawmakers over whether to request troops stay beyond the deadline.
The U.S. military has said the Shiite-militias -- Kataib Hezbollah, Asaib al Haq, and the Promise Day Brigade -- are using the bombings to try to take credit for driving American forces out of the country.
"If anything, these groups have proven to be resilient and we have no doubt they will continue to conduct violent attacks in the months ahead," Buchanan said.
He said Iraq's best weapon against the violence was to continue targeting the groups.
CNN's Fattah Mahmood Fattah contributed to this report.