BAGHDAD - Iraqi militia fighters are being trained in Iran to build and use deadly armor-piercing roadside bombs and complex attack strategies against American forces, the US military said Wednesday.

In this image released by the US Army on Wednesday April 11, Ryan Crocker, United States ambassador to Iraq, jokes with patrons of a market in Dora neighborhood in Baghdad, Iraq, Thursday, April 5, 2007. [AP]

US military spokesman Maj. Gen. William Caldwell would not say how many militiamen had gone to Iran but said that questioning of fighters captured as recently as this month confirmed many had been in Iranian training camps.

"They do receive training on how to assemble and employ EFPs," Caldwell said, adding that fighters also were taught how to carry out attacks that use explosives followed by assaults with rocket-propelled grenades and small arms.

EFP stands for explosively formed penetrator. The weapon causes great uneasiness among US forces because it explodes with tremendous force and can penetrate heavily armored vehicles with a fist-size lump of molten copper. In January, US officials said EFPs had killed at least 170 American soldiers in Iraq.

"We know that they are being in fact manufactured and smuggled into this country, and we know that training does go on in Iran for people to learn how to assemble them and how to employ them. We know that training has gone on as recently as this past month from detainees' debriefs," Caldwell said at a weekly briefing.

The general would not say specifically which arm of the Iranian government was doing the training but called the instructors "surrogates" of Iran's intelligence agency. He also said the US military had evidence that Iranian intelligence agents were active in Iraq in funding, training and arming Shiite militia fighters.

Caldwell opened the briefing by showing photographs of what he said were Iranian-made mortar rounds, RPG rounds and rockets that were found in Iraq.

The accusations are the latest attempt by the United States to show that Iran is meddling in the Iraq war. If true, the training poses a serious threat to both US forces and Iraqi stability. Iraq, which like Iran is majority Shiite, has found itself in a difficult position since the US-led invasion in 2003, trying to maintain good relations with its neighbor while not angering the Americans.

Commanders of a splinter group inside the Shiite Mahdi Army militia have told The Associated Press there are as many as 4,000 members of their organization that were trained in Iran and that they have stockpiles of deadly roadside bombs known as EFPs.

Asked for reaction, Caldwell said he could not confirm the number.

The Mahdi Army commanders who spoke to the AP did so on condition of anonymity because their organization is viewed as illegal by the American military and giving their names would likely lead to their arrest and imprisonment. They said Iran's elite Revolutionary Guards was running the training operation in Iran.

Gen. Ramazan Sharif, spokesman of the Revolutionary Guards, denied ties with the Mahdi Army in Iraq.

"This sort of news and information is planned by occupier (US) forces in Iraq as part of their psychological operations against Iran," he said.

"This hollow claim was repeatedly rejected by Iraqi government and officials. And the occupiers could not provide any evidence to support it," Sharif said. He said the United States was using such claims as a cover for its failures in Iraq.

The Mahdi Army is loyal to radical Iraqi cleric Muqtada al-Sadr, who US officials say is holed up in Iran.

On Wednesday, Iraqi Cabinet ministers allied to al-Sadr threatened to quit the government to protest the prime minister's lack of support for a timetable for US withdrawal.

Such a pullout by the bloc that put Prime Minister Nouri al-Maliki in office could collapse his already perilously weak government. The threat comes two months into a US effort to pacify Baghdad in order to give al-Maliki's government room to function.

At least 28 people were killed or found dead across Iraq on Wednesday.

The US military announced two more soldier deaths: One soldier was killed and two were wounded by a roadside bomb Wednesday in an eastern section of the capital, and another soldier died a day earlier in an attack in southern Baghdad. One soldier was wounded in that incident.

At least 3,287 members of the US military have died since the beginning of the war in 2003, according to an AP count. The figure includes seven military civilians.