Five US marines were killed in Iraq when their vehicle struck a bomb near the western town of Ramadi yesterday.
A US military statement gave no details of yesterday's attack, but it was the second time in a week that insurgents have inflicted a high death toll on marines in the same area in a single blast.
US officers have said some rebels have been using "shaped charges", which concentrate blasts onto a small area allowing them to penetrate even the heavy armour of tanks.
Improvised explosive devices, as the troops call them, are the biggest single threat to Americans in Iraq; although many are spotted and defused, dozens go off every day.
Five marines were killed in a roadside bomb blast on their vehicle near Haqlaniya, just outside Ramadi, last Thursday.
Since the US-led invasion in March 2003, at least 1,708 US soldiers and an unknown number of Iraqis have been killed in Iraq.