Gates said the US side was surprised when Iranians were captured in sweeps aimed at insurgent arms supply networks in Iraq but not that Iran was involved in providing the more sophisticated "explosively formed projectiles."
"I think Iran is very much involved in providing either the technology or the weapons themselves of these explosively formed projectiles," he said.
"Now they don’t represent a big percentage of the IED attacks but they are extremely leghtal," he said.
The bombs have shaped charges that can penetrate the armor of an M-1 Abrams battle tank, a level of technical sophistication that Gates said pointed to an Iranian source.
"I think there are some serial numbers, there may be some markings on some of the projectiles, fragments that we’ve found," he told reporters here.
"I’m just frankly not specifically certain myself of the details. But I understand there is pretty good evidence tying these EFPs to the Iranians.
The US military had been preparing to go public with the evidence to back its frequent charges that Iran has been supplying Shia insurgent groups with weapons and training to attack US forces.
But a briefing that had been scheduled for reporters in Baghdad was postponed, raising questions about how solid the evidence was.
The Bush administration also has been moving to lower rising tensions with Iran which have been stoked by the deployment of a second aircraft carrier strike group to the Gulf and the capture of Iranians in raids in Iraq.
They have come on top of a mounting diplomatic confrontation over Tehran’s nuclear programme.
"My impression frankly over the last few weeks there has been an effort in Washington to tone down everybody else," Gates said Friday. "I don’t know how many times the president, the secretary of state and I have had to say we have no intention of attacking Iran."
Iran’s supreme leader Ali Khamenei on Thursday vowed retaliation against US interests worldwide if the United States attacked Iran. Earlier this week, the Iranian Revolutionary Guard tested an anti-ship missile in an apparent reminder of the risks to US warships in the Gulf.
But Gates shrugged off the Iranian posturing as "just another day in the Persian Gulf."
However he acknowledged Friday that the announcement of the carrier deployment and the raids in Iraq had caused a stir.
But he said US forces were not specifically targeting Iranians in those raids. "Some of us were surprised that they actually did find some Iranians involved in that," he said.