BAGHDAD — U.S. defense and intelligence officials sought Sunday to bolster the charge that Iran was providing arms to Shiite Muslim militants in Iraq, displaying munitions and weapons fragments that they said constituted evidence that Tehran was contributing to Iraq's violence.
They also alleged that a group under the command of Iran's supreme leader was behind the smuggling of weapons across the Iran-Iraq border.
The briefing, held under unusually secretive circumstances, featured three U.S. officials, none of whom would be identified, and two tables laden with what they said were uniquely Iranian military hardware and weapons fragments.
The officials said an Iranian weapon known as an Explosively Formed Penetrator had been responsible for the deaths of about 170 of the 3,400 U.S.-led forces killed in Iraq. The armor-piercing devices are used in roadside bomb attacks, which have increased in the last year, the officials said.
The presentation came as tensions continued to mount between Washington and Tehran over Iran's nuclear ambitions and regional aspirations.
With two U.S. warship groups in the Persian Gulf, the allegations raised suspicion that the Bush administration was trying to build a case for war, much as it had used intelligence reports to win support for the U.S.-led invasion of Iraq.
"That's how we got into the mess in Iraq," Sen. Christopher J. Dodd (D-Conn.) said on CBS television. "That's why some of us supported those resolutions, because of doctored information. So I'm very skeptical based on recent past history about this administration."
Some experts wondered whether it was possible to discern the intended uses of such weapons in a situation as complex as Iraq's.
"There is a virtual civil war happening," said Wayne White, a former State Department intelligence analyst who is a scholar at the Middle East Institute, a Washington think tank. "If Iran is passing munitions to Shia militias, it could be more in the context of the ongoing sectarian strife than aimed at the U.S."
The U.S. officials said they wanted to fend off an alarming new type of weapon that was inflicting an increasing number of casualties among American and Iraqi troops. The presentation was a scaled-back version of one postponed two weeks ago amid a dispute within the administration over the strength of the evidence.
The briefing seemed deliberately limited. The officials appeared to back away from previous U.S. claims that Iran, a mostly Shiite country, was supporting the Sunni Arab insurgents who have by far killed the largest number of U.S. troops.
Instead, the officials alleged that Shiite groups ostensibly loyal to radical anti-American cleric Muqtada Sadr were involved in the smuggling and use of the weapons.
Few independent analysts think Iran or any other country is playing a decisive role in the sectarian warfare and insurgent violence engulfing at least eight of Iraq's 18 provinces.
But many Western intelligence and Middle East experts think Tehran is pursuing a policy of "managed chaos" in Iraq. They suggest Iran is supporting its Shiite and Kurdish allies, who dominate Iraq's new government, while contributing to the violence in an effort to keep the U.S. preoccupied.
Within Iraq, some saw the presentation as a defensive maneuver meant to drive a wedge into the increasingly cozy relationship between the Shiite-led governments in Baghdad and Tehran.
"The Iraqi government doesn't think Iran is the enemy," said Mahmoud Othman, a Kurdish lawmaker in Baghdad. "The Americans are trying to convince the Iraqis that they're not just against Iranians because of the nuclear file, but because of what's happening here in Iraq."
One Shiite lawmaker said she wondered whether similar presentations would be made suggesting Saudi, Jordanian and Syrian interference in Iraq's affairs. Young Arab men launch most of the suicide bombings targeting Iraqi civilians.
"This is not solid evidence that these weapons came from the Iranian government," said Neda Sudani, a Shiite lawmaker loyal to Sadr. "These could be arms brought by smugglers for profit."
The new allegations may place Iraqi Prime Minister Nouri Maliki in an increasingly uncomfortable position as he tries to nurture a relationship with Tehran without angering the Bush administration.
Iraq's blossoming partnership with Iran has included discussions on security and intelligence matters and on offers to train and equip Iraqi forces. Iraq's interior minister, who works closely with U.S. intelligence and defense officials, met Saturday with the Iranian ambassador to Iraq, a former member of the Revolutionary Guard.
Several Iranian officials contacted by phone Sunday evening in Tehran declined to comment on the U.S. presentation. In recent weeks, Iranian officials have accused the Bush administration of unfairly using Iran as a scapegoat for its policy failures in Iraq.