Insurgents in Iraq have increased the amount of explosives they pack into bombs targeting US army patrols, a top Pentagon official has said.
The Bradley is one of the most heavily protected US vehicles
Brig Gen David Rodriguez was speaking after a roadside bomb killed seven US soldiers in Baghdad on Friday - the highest US death toll so far this year.
Troops were encountering fewer bombs, he said, but they were more powerful.
A "multi-pronged effort" - gathering intelligence on bombs and adapting tactics - could reduce losses, he said.
Brig Gen Rodriguez warned, however, that boosting armoured protection for US vehicles was not enough in every case.
The seven soldiers killed on Friday were travelling in a Bradley armoured fighting vehicle - the most heavily-protected US vehicle after the Abrams tank.
US soldiers and their families have complained the military has not given their vehicles adequate protection in Iraq.
Earlier this week, Maj Gen Peter Chiarelli, a US commander, said his forces were having greater success in finding and defusing roadside explosives and car bombs.
But, he warned, stopping them altogether required more co-operation from ordinary Iraqis.