An American intelligence assessment described to The New York Times said that “as part of its strategy in Iraq, Iran is implementing a deliberate, calibrated policy — approved by Supreme Leader Khamenei and carried out by the Quds Force — to provide explosives support and training to select Iraqi Shia militant groups to conduct attacks against coalition targets.” The reference was to Ayatollah Ali Khamenei, the Iranian leader, and to an elite branch of Iran’s Islamic Revolutionary Guards Command that is assigned the task of carrying out paramilitary operations abroad.

“The likely aim is to make a military presence in Iraq more costly for the U.S.,” the assessment said.

Other officials believe Iran is using the attacks to send a warning to the United States that it can inflict casualties on American troops if the United States takes a more forceful posture toward it.

Iran has publicly denied the allegations that it is providing military support to Shiite militants in Iraq. Javad Zarif, Iran’s ambassador to the United Nations, wrote in an Op-Ed article published on Thursday in The Times that the Bush administration was “trying to make Iran its scapegoat and fabricating evidence of Iranian activities in Iraq.”

The explosively formed penetrator, detonated on the roadside as American vehicles pass by, is capable of blasting a metal projectile through the side of an armored Humvee with devastating consequences.

American military officers say that attacks using the weapon reached a high point in December, when it accounted for a significant portion of Americans killed and wounded in Iraq. For reasons that remain unclear, attacks using the device declined substantially in January, but the weapons remain one of the principal threats to American troops in and around Baghdad, where five additional brigades of American combat troops are to be deployed under the Bush administration’s new plan.

“It is the most effective I.E.D out there,” said Lt. Col. James Danna, who led the Second Battalion, Sixth Infantry Regiment in Baghdad last year, referring to improvised explosive devices, as the roadside bombs are known by the American military. “To me it is a political weapon. There are not a lot of them out there, but every time we crack down on the Shia militias that weapon comes out. They want to keep us on our bases, keep us out of their neighborhoods and prevent us from doing our main mission, which is protecting vulnerable portions of the population.”