 The weapons were captured in a Sunni district last week, the US said |
Sunni militants are being armed with Iranian-made munitions, US military spokesman Maj Gen William Caldwell told reporters in Baghdad.
These include mortar rounds and rocket-propelled grenades, he said.
There was no immediate reaction from the government in mainly Shia Iran which has been accused of arming fellow Shia militants in Iraq in the past.
Training claim
The weapons, which were shown at the news conference, were discovered in a car in a Sunni district of Baghdad last week, the Americans said.
 Gen Caldwell said Sunni militants were getting "skilled training" |
Gen Caldwell said the Iranians were not only supplying weapons to unspecified groups fighting the coalition and Iraqi government forces but training them too.
"There are groups that are receiving training in Iran with the most modern weapons and munitions that are available and then being smuggled into Iraq and being utilised by these groups against the Iraqi security force and coalition forces," he said.
"That required some very skilled training to be able to use them and employ them like they were being used."
Iran threat
Gen Caldwell also accused the Iranians of helping Iraqi militants use roadside bombs, which have been used to devastating effect in ambushes on US and coalition forces.
The devices have so far killed more than 170 US soldiers since the Iraq invasion in 2003.
The BBC's Jim Muir says the Iraqi government is hoping a planned conference in Egypt next month will defuse tensions with its neighbours, and perhaps even start a reconciliation process between the Americans and Iran.
But now Iran is threatening to pull out of the talks, as they are demanding the release of five Iranian officials seized by the Americans from an office in the Kurdish area of northern Iraq in January, our correspondent says.
The US on Wednesday ruled out freeing the five, who it accuses of meddling in Iraqi affairs.
The White House also denied Iranian state television reports it tortured a diplomat held in custody for two months.
