Powerful roadside bombs believed to be of Iranian origin and considered the signature weapon of Shiite militias are turning up in the hands of Sunni insurgents south of Baghdad, said Maj. Gen. Rick Lynch, commander of the US Army's Task Force Marne. "We're seeing Iranian EFPs in the hands of Sunni extremists," Lynch told a group of Western reporters. But Lynch, whose command covers the southern rim of Baghdad and mostly Shiite areas to the south, said it was unclear whether the Iranians were supplying the weapons directly or whether the Sunnis were buying them on the black market. Roadside bombs, which the military calls "improvised explosive devices" or IEDs, have long been the major killer of American soldiers. The US military has upgraded the armor on Humvees and taken over measures to protect troops against those bombs. But some of those measures are ineffective against EFPs, which hurl a fist-sized lump of molten copper through the toughest American armor. Insurgents are also burying some bombs deep enough to avoid US detection devices, Lynch said. Such measures point to the resilience of both Sunni and Shiite extremist groups, which Lynch called a "thinking enemy," capable of responding to American countermeasures. US military officials have been saying for months that the Iranians were supplying EFPs to Shiite militias, despite strong denials by the Tehran government. Lynch would not speculate why the Iranians would want to deal with armed groups opposed to the Shiite majority which Tehran wants to see maintain a dominate position in Iraq. Nevertheless, Lynch said there were clear signs of Iranian influence among both Sunni and Shiite extremists groups although he would not elaborate.