A report on deadly airstrikes in Afghanistan calls for better training for air and ground forces to reduce civilian casualties that have undermined the counterinsurgency campaign, The Associated Press said.

The recommendation on training and a second one urging a review of the use of air support are among a half-dozen responses in an unreleased report on a May 4 bombing that killed dozens of Afghan civilians, two Defense Department officials said Wednesday. They spoke on condition of anonymity because the report has not been made public.

A separate recommendation is to review aircraft used for air support that troops call in to back them up, officials said.

The report recommends that U.S. air and ground forces headed to Afghanistan should receive advance training in the kinds of scenarios they could face, including situations that have resulted in civilian deaths, one official said. The report suggests periodic refresher training throughout troops' tours of duty in the war zone, meaning that forces would get training early and often to reinforce battlefield tactics.

The Obama administration is sending 21,000 troops to Afghanistan to bolster the nearly 8-year-old campaign against the Taliban.

Defense Secretary Robert Gates and other top military officials have said that reducing civilian deaths is crucial to winning Afghan support for American and coalition operations against the insurgents and boosting the population's support for the central government.

Afghan officials have said that 140 civilians were killed during the airstrikes in Farah province. The report being prepared for release holds to earlier U.S. estimates of a lower death toll of about 30 civilians and 60 to 65 Taliban fighters. Officials acknowledged that a precise number may never be known because many victims were buried before the investigation started.

Defense officials have struggled for days to produce an unclassified summary that can be released publicly and would explain the findings of the investigation. They have also worked to pare down hours of video that shows what happened.

But defense officials have acknowledged mistakes in the airstrikes, saying some tactics and procedures were not followed strictly. The review found that the early May airstrikes were carried out against legitimate Taliban targets but that, at least in one case, it was unclear whether civilians were in the vicinity.

In one airstrike, an Air Force B-1 dropped a 2,000 pound bomb on a building that Taliban members had been seen entering, officials have said. Some military officials have questioned whether that bomb was larger and more destructive than warranted.

The air crew received permission to strike, but circled around and dropped the bomb without reconfirming. That lag time, officials said, may have allowed the insurgents to leave the targeted building and civilians to enter in the interim before the blast.

It is not known whether that airstrike added to the civilian toll, officials have said.

Officials said the report does not recommend changes in tactics and procedures used in Afghanistan, though the new commander there, Lt. Gen. Stanley McChrystal, has said he will review them as well as all existing rules of engagement.

Military commanders have reviewed and rewritten such guidelines — including those on bombing missions and on how special forces operate — in an effort to avoid Afghan casualties. Rules that were tightened in a review late last year may not have been followed by troops on May 4, officials have said.

According to the U.S. military, the battle in Farah province began a day after Taliban fighters entered two villages, demanded money from civilians and killed three former government employees. An Afghan force rushed in, only to be ambushed by as many as 300 insurgents.

The provincial governor asked for U.S. military help, and American ground troops joined the battle, the U.S. officials said. Before the battle was over, troops called in F-18 fighter jet airstrikes as well as help from the B-1 bomber, coordinating with the ground commander to strike a half-dozen targets, including buildings and a tree grove that insurgents were firing from or massing in, the U.S. officials have said.