By David Morgan

WASHINGTON, Feb 7 (Reuters) - Sunni Arab insurgents in Iraq have developed their own crude versions of the Iran-originated armor-piercing munitions that have been a hallmark of Shi'ite militant roadside attacks on U.S. troops, U.S. defense officials say.

The devices are a form of weaponry known as explosively formed penetrators, or EFPs, which have given Shi'ite militants the firepower to penetrate the heaviest U.S. armor. Iran denies U.S. assertions that it has supplied EFPs to anti-U.S. Shi'ites.

So far, the Sunni versions, first discovered in 2007, have been crude home-made knock-offs of the Iranian-made EFPs, according to U.S. officials.

But officials are concerned that Sunni EFPs could become more deadly if they follow the same path toward sophistication as the most common roadside bombs, known as improvised explosive devices, or IEDs, used throughout Iraq against U.S. forces for much of the war..

"That's a fairly typical trajectory. I would expect (EFPs) to evolve," said Stephen Biddle of the Council on Foreign Relations.

U.S. officials believe the new EFPs are the work of either al Qaeda in Iraq or the Islamic State in Iraq, related networks of Sunni Islamist militants who have recently come under fire from a joint U.S.-Iraqi counterinsurgency operation.

The munitions have shown up in Sunni areas at a time when Islamist groups are particularly vulnerable after being driven from safe havens by U.S. and Iraqi troops aided by U.S.-allied Sunni tribesmen.

INEFFECTIVE FOR NOW

But unlike Shi'ite EFPs, U.S. officials say the Sunni versions have proved ineffective, for now.

"They're very poorly formed," said Army Maj. Gen. Mark Hertling, the U.S. commander in northern Iraq, where as many as 10 Sunni-made EFPs have been discovered.

"They're locally made, certainly not imported," he told Reuters in a recent interview in Iraq.

Defense officials declined to say whether the Sunni EFPs have been used against U.S. forces or have caused casualties.

The deadly effectiveness of the more common IEDs prompted the Pentagon to launch a $22.4 billion program to equip U.S. forces in Iraq with trucks, personnel carriers and patrol vehicles designed to resist the force of the explosion.

But U.S. Mine Resistant Ambush Protected vehicles, known as MRAPs, are still vulnerable to EFPs, which are designed drive metal projectiles into armored vehicles.

EFP attacks spiked upward during the first two weeks of January but have since returned to low levels seen late in 2007.

"The predominant EFP threat remains those of Iranian origin, which are still being used by rogue criminal Shi'ite elements," said Army Maj. Winfield Danielson, a spokesman for multinational forces in Iraq.

Officials have disclosed no casualty figures for EFP attacks, other than to say early last year that 170 U.S. troops had been killed by Shi'ite weapons. (Additional reporting by Paul Tait in Baghdad, editing by Philip Barbara)

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