Afghan officials say the strike killed 140 civilians, making it the most deadly military operation involving civilians since the 2001 US-led invasion.
Preliminary US estimates had put the civilian death toll at between 20 and 35 and the number of Taliban fighters killed at 80 to 95.
An Afghan rights watchdog has said that 97 civilians died, and that no more than two Taliban fighters were killed.
"There are some estimated but ... I don't think we ever will really know how many," Mullen said.
Officials have already said some mistakes were made and that procedures were not strictly followed in the operation, media reports said.
|The US is sent to send thousands more troops
to Afghanistan [Reuters]
The report, details of which have been leaked to the US media, urges increased training for US troops.
It is also set to urge a review of the use of air support when targeting Taliban fighters after mounting civilian casualties that have provoked outrage in Afghanistan.
It also recommends that US forces sent to Afghanistan should receive training in the kinds of scenarios they could face, including situations that have resulted in civilian deaths, one official told the Associated Press.
Periodic refresher training throughout soldiers' tours of duty in the war zone, is also set to be recommended by the report.
The review said the early May air raids were carried out against Taliban targets in Farah but that, at least in one case, it was unclear whether civilians were in the vicinity.
In one incident, an Air Force B-1 dropped a 900kg bomb on a building that Taliban fighters had been seen entering, officials say.
Some military officials have questioned whether that bomb was larger and more destructive than warranted, the Associated Press reported.
|Afghans have expressed outrage at
civilian deaths from US bombing [Reuters]
The air crew received permission to strike, but circled around and dropped the bomb without reconfirming, the report is set to say.
That delay, officials said, may have allowed the fighters to leave the building and civilians to enter before the bomb was dropped.
It is not known whether the raid 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, Lieutenant-General Stanley McChrystal, has said he will review them as well as all existing rules of engagement.
The US government has come under increasing criticism over the past year for civilian deaths during operations against Taliban and al-Qaeda fighters in Afghanistan and Pakistan.
Barack Obama, the US president, and other Washington officials have repeatedly expressed regret for civilian casualties and vowed to take measures that would avoid future attacks.