BAGHDAD — A massive U.S. aerial bombing campaign launched in Iraq on Thursday attempted to strike a delicate balance — routing members of a newly resurgent al-Qaida while trying to avoid civilian casualties that could alienate ordinary Iraqis.

U.S. planes attacked a rural Sunni area southeast of Baghdad with 40,000 pounds of bombs during a 10-minute period. That surpassed the tonnage that previously had been dropped there during an average month, said Maj. Alayne Conway, a U.S. military spokeswoman.

The attacks targeted suspected al-Qaida weapons caches, supply lines and bomb-making sites, Conway said. No civilian casualties were immediately reported, she said, reflecting a central focus of the U.S. military's year-old counterinsurgency strategy: winning the support of the local population.

U.S. commanders have repeatedly cited better cooperation from Iraqis as a main reason for a dramatic drop in violence since last summer.

"You saw a lot more damage to the civilian population in 2004 than you're seeing now. Even though you have a huge uptick in offensive operations, it looks like the military is taking greater care not to harm civilians," said Colin Kahl, a security studies professor at Georgetown University.

The airstrikes are part of a broad U.S. offensive launched this week to counter al-Qaida in Iraq, which had been showing signs of a revival in Baghdad and elsewhere amid a spate of recent suicide bombings.

Iraqi casualty statistics are not regularly provided by the U.S. military or the Iraqi government, and can be difficult for other groups to track with precision.

Iraq Body Count, a British non-governmental organization that has compiled casualty figures based primarily on media reports since the war started, says U.S. forces caused an average of 63 Iraqi civilian deaths per month in 2007 — down from 169 per month in 2004. The apparent decline in civilian casualties comes despite a greater U.S. troop presence in Iraq during the past year and an increase in the emphasis of airpower.

There were 1,119 airstrikes through mid-December, according to U.S. Central Command Air Forces — about five times the number in 2006.

The U.S. military has become more selective in choosing its targets, taking care to avoid those that might cause heavy civilian casualties.

"The planning has gotten a lot better," said Maj. Joe Edstrom, a military spokesman.