In the span of 10 minutes Thursday, U.S. warplanes dropped as much explosive south of Baghdad as they usually do in a month, a thundering barrage of more than 40,000 pounds of bombs intended to blow up insurgents' weapons stashes.

The B-1 bombers and F-16 fighter jets dropped 38 bombs in the opening minutes of the operation, which was aimed at three main areas of Arab Jubour, a rural district on the outskirts of the capital that became a focal point of the U.S. military troop buildup last year.

"This is al Qaeda in Iraq, one of their last safe havens in our area of operations," said Sgt. 1st Class Randal Maynard, a U.S. military spokesman. "And we're going in, choking them out from our region."

The bombing campaign, which targeted caches of roadside bombs first identified by surveillance drones, was the most intensive aerial bombardment in the southern region. It came as part of the military's overall offensive, known as Phantom Phoenix, under way now in several parts of the country. While ground forces continued to pursue insurgents in Diyala province north of Baghdad, the warplanes tried to clear the southern territory of the bombs that have regularly destroyed U.S. armored vehicles.

"These were some big IEDs buried in the ground," Maynard said, using the military abbreviation for improvised explosive device, or roadside bomb. "Had the soldiers drove up on these IEDs, it could have caused six to eight deaths."

Maynard said there was no immediate estimate of how many people died in the bombings, because the U.S. military has not yet explored the area on the ground to "validate any kills."

Arab Jubour lies along the Tigris River amid lush tracts of palm groves, fields and grasslands. Before the U.S. military troop buildup last year, it had become an essentially ungoverned enclave, devoid of Iraqi police officers and dominated by Sunni insurgents. As U.S. soldiers attempted to crack down on the rural lands around Baghdad, they erected a makeshift base there and sent regular patrols down the often bomb-riddled roads.

These operations, along with the rise of Sunni volunteer forces aligned with U.S. soldiers, have been followed by a sharp drop in violence. But commanders regularly say that al Qaeda in Iraq still keeps a foothold in the area, and the bombing operation was a sign that they are still seen as a threat to the U.S. military.

This article appeared on page A - 10 of the San Francisco Chronicle