Army Brigadier General David Rodriguez on Friday said the trend of using more powerful improvised explosive devices (IEDs) has become prominent over the past two weeks.
"We have noticed in the recent couple of weeks that the IEDs are all being built more powerfully, with more explosive effort in a smaller number of IEDs," Rodriguez told reporters at a Pentagon briefing.
His comments came a day after a roadside bomb targeted a US military convoy in Baghdad, killing seven soldiers.
"There are a lower number of IEDs, but they have been more powerful," Rodriguez said.
The brigadier could offer no explanation for the new tactic, including whether or not those fighting US troops were using new experts to cobble together explosive shells and other devices.
But he said the military was using a broad effort to detect and defeat IEDs by studying enemy tactics, shifting US military procedures and boosting intelligence efforts.
"We will continue to watch that carefully," he said.
US Department of Defence spokesman Lawrence Di Rita noted that US commanders in Iraq had said more IEDs were being discovered before they could explode.