BAGHDAD (Reuters) -Five U.S. marines were killed in Iraq when their vehicle struck a bomb near the violent western town of Ramadi, the U.S. military said on Thursday.

Its statement gave no details of Wednesday”s attack but it was the second time in a week that insurgents have inflicted such a high death toll on marines in the same area in a single blast; it strengthens the impression that guerrillas have developed more sophisticated bombing techniques.

U.S. officers have said some rebels have been using &#34shaped charges,&#34 which concentrate blasts onto a small area allowing them to penetrate even the heavy armor of tanks.

Improvised explosive devices, as the troops call them, are the biggest single threat to Americans in Iraq. Though many are spotted and defused, dozens go off every day.

That has limited U.S. troops” freedom of movement and forced the military to improve the armor on all its vehicles, at considerable cost in both equipment and in added fuel bills for the heavier loads.

Five marines were killed in a roadside bomb blast on their vehicle near Haqlaniya, just outside Ramadi, last Thursday.

The same marine division said a sixth service member also died on Wednesday — a sailor who was shot in Ramadi. Most sailors attached to U.S. marine units are medics.

Since the U.S.-led invasion in March 2003, at least 1,709 U.S. defense personnel have been killed in action in Iraq.