U.S. military officials say the deaths of 21 U.S. Marines near the Iraqi town of Haditha this week are the result of an ongoing offensive against insurgents who use that town, and others in the area, as bases for their operations. The explosion along a roadside in the early morning hours Wednesday was the deadliest single attack on U.S. forces since December. Fourteen Marines died, along with their Iraqi interpreter, even though they were riding in an armored vehicle.

U.S. officials say such roadside bombs continue to be the insurgency's most lethal weapon. The U.S. military's deputy director for regional operations, Brigadier General Carter Ham, says while the number of such bombs is declining, they tend to be larger than they used to be and are "very, very lethal." General Ham says the insurgents are making other changes, too.

"We are seeing different techniques that are being used in an effort to counter the efforts of coalition and Iraqi security forces to protect folks while they are moving, different types of penetrators, different techniques of triggering the events," he said. "This is a very brutal, lethal and adaptive enemy."

On Monday, another U.S. Marine was killed in a bombing near the same town, and that same day in the same area six Marines died in gun battle with insurgents. General Ham reports that the series of attacks in and around Haditha appears to be related to a new offensive being conducted by coalition forces in western Iraq in traditional insurgent strongholds. The general says the troops are securing several towns along the Euphrates River northwest of Baghdad, including Haditha, and limiting the insurgents' ability to move around the area.

"Perhaps previously they may have had an opportunity to move," he said. "For example, if there was pressure in Haditha, they could perhaps move someplace else. Well, now because of the simultaneity of operations that Multi-National Force-West is conducting they don't have that freedom of movement. And I think that's one of the contributing causes to the number of direct contacts that occurring."

General Ham says additional coalition forces have been brought to the area from other parts of Iraq, and that the operation involves about 1,000 Iraqi troops as well.