This image provided by the U.S. military shows what officials call explosively

Baghdad, Iraq ? U.S. military officials on Sunday accused the highest levels of the Iranian leadership of arming Shiite militants in Iraq with sophisticated armor-piercing roadside bombs that have killed more than 170 American troops from the American-led coalition.

The military command in Baghdad denied, however, that any newly smuggled Iranian weapons were behind the five U.S. military helicopter crashes since Jan. 20 – four that were shot out of the sky by insurgent gunfire.

A fifth crash tentatively has been blamed on mechanical failure. In the same period, two private security company helicopters also have crashed but the cause was unclear.

The deadly and highly sophisticated weapons the U.S. military said it traced to Iran are known as “explosively formed penetrators,” or EFPs.

The presentation was the result of weeks of preparation and revisions as U.S. officials put together a package of material to support the Bush administration’s claims of Iranian intercession on behalf of militant Iraqis fighting American forces.

Senior U.S. military officials in Baghdad said the display was prompted by the military’s concern for “force protection,” which, they said, was guaranteed under the United Nations resolution that authorizes American soldiers to be in Iraq.

Three senior military officials who explained the display said the “machining process” used in the construction of the deadly bombs had been traced to Iran.

The experts, who spoke to a large gathering of reporters on condition that they not be further identified, said the supply trail began with Iran’s Revolutionary Guards Quds Force, which also is accused of arming the Hezbollah guerrilla army in Lebanon. The officials said the EFP weapon was tested there first.

The officials said the Revolutionary Guard and its Quds force report directly to Iran’s supreme leader, Ayatollah Ali Khamenei.

The briefing on Iran was revised heavily after officials decided it was not ready for release as planned last month.

Senior U.S. officials in Washington – cautious after the drubbing the administration took for the faulty intelligence leading to the 2003 Iraq invasion – had held back because they were unhappy with the original presentation.

The display appeared to be part of the White House drive that has empowered U.S. forces in Iraq to use all means to curb Iranian influence in the country, including killing Iranian agents.

It included a handful of mortar shells and rocket-propelled grenades that the military officials said were made in Iran.

The centerpiece of the display, however, was a gray metal pipe about 10 inches long and 6 inches in diameter, the exterior casing of what the military said was an EFP, the roadside bomb that shoots out fist-sized wads of nearly molten copper that can penetrate the armor on an Abrams tank.