U.S. and other coalition officials in Iraq are voicing concern about the recent discoveries of highly-sophisticated and lethal roadside bombs being smuggled into the country from Iran.

Speculation about whether insurgents in Iraq were now using so-called "shape charges" as roadside bombs, reached new heights after Wednesday's deadly attack against a U.S. Marine vehicle in the western town of Haditha.

Fourteen Marines were killed after their amphibious assault carrier hit a bomb so powerful, it flipped the 27 metric ton vehicle into the air.

On Thursday, the top spokesman for Iraq's multi-national forces, U.S. Brigadier General Donald Alston, told reporters that he did not know what type of bomb was used against the Marines.

VOA has now learned that the improvised explosive device used in Wednesday's attack was not a shape charge, but an ordinary, albeit a very large, bomb.

A senior coalition military official in Baghdad, who spoke to VOA on the condition of anonymity because of the sensitive intelligence involved, says as devastating as the bombing was in Haditha, shape charges are far more worrying to U.S. troops because they are not just designed to destroy vehicles, but to penetrate armor.

A shape charge combines an explosive charge with metal. The explosive is shaped to concentrate the blast, which then turns the metal into a high speed slug that can rip through the heaviest steel doors.

Military commanders say this technical expertise was not seen in Iraq when the insurgency began two years ago. But they confirm that shape charges have been used in attacks in the past several months, killing and wounding several U.S. troops.

There is also evidence that these more sophisticated devices are flowing freely into Iraq across its porous with Iran.

Details are still vague, but the senior coalition official tells VOA that Iraqi border guards there have recently intercepted several shipments, which have included fully manufactured shape charges as well as components for making them.

The latest discovery occurred on July 20th. The senior official says that shipment contained four devices, which, like the other shape charges seen in Iraq, closely match those used by the Iranian-backed Shi'ite Hezbollah group in the 1980s in its war to drive Israel out of southern Lebanon.

The coalition official here says there is no evidence to suggest that the government in Tehran is facilitating the smuggling of shape charges into Iraq. But U.S. officials say they believe the shipments may have been sent with the backing of the Iranian Revolutionary Guard.

Military intelligence officials say what may be happening is that the technology for making shape charges is spreading among a variety of Iraqi insurgent groups. Coalition forces know of at least one insurgent cell in Baghdad, which has been attempting to make shape charges locally.