WASHINGTON, Aug. 4 (Xinhuanet) -- In recent months, roadside bombs used by Iraqi militants have become more sophisticated and more deadly, posing a growing challenge to US soldiers, The New York Times reported Thursday.

    The explosion that killed 14 US Marines in western Iraq Wednesday was powerful enough to flip the 25-ton amphibious assault vehicle they were riding in, US military officers told the newspaper.

    In fact, the new problem facing the US military in Iraq emerged over a week earlier, on July 23, when a huge bomb buried on a road southwest of Baghdad Airport detonated underneath a Humvee carrying four American soldiers, killing all of them instantly.

    The explosive device was constructed from a bomb weighing 500 pounds or more and the blast left a crater 6 feet deep and nearly 17 feet wide.

    What happened soon after the July 23 attack further frustrated the US military.

    A British explosives expert, part of a special squad formed to investigate major insurgent bomb attacks, stepped on a second, smaller bomb buried near the first and was badly wounded.

    He later had an arm and a leg amputated. A third device, hidden a few yards away, was found and defused.

    To counter the growing threat from roadside bombs, the Pentagonhas dispatched 24,000 armored Humvees to Iraq since late 2003.

    But the militants have responded by building bombs powerful enough to penetrate the vehicles' steel plating.

    US experts said there are plenty of evidence that Iraqi militants are increasingly carrying out the so-called "shaped" charges, which concentrate the blast and give it a better chance of penetrating armored vehicles, causing higher casualties.

    They admitted that the roadside bomb attacks can't be stopped and what the US military can do is to reduce the number of attacks, which average around 65 a day against US and Iraqi troops at present. Enditem