A US military investigation into the August crash of a helicopter in Afghanistan that killed 30 Americans has found no major mistakes were made in the operation, officials said Wednesday.

The probe concluded that senior officers made "appropriate" decisions in deploying a large contingent of special operations forces on a Chinook helicopter to back up ground troops in a firefight, two defense officials told AFP.

Taliban fighters shot down the Chinook southwest of the capital, in the deadliest single incident of the war for US forces.

The report found "there was no problem with the mission, no problem with the equipment" and no commander was singled out for possible punishment, said one of the defense officials, who spoke on condition of anonymity.

The attack dealt a blow to the elite US special forces, with the loss of 22 Navy SEAL commandos and three Air Force special operators. Most of the Navy commandos came from the same SEAL team credited with killing Al-Qaeda chief Osama bin Laden.

Military officials were briefing families of the troops killed and lawmakers on the report's findings, which could be released later Wednesday.

In the August incident, the CH-47 helicopter was sent in to bring reinforcements as US forces went after insurgent leaders in the Tangi Valley of Wardak province.

But the helicopter was shot out of the sky with a rocket-propelled grenade, killing all 38 people on board, including seven Afghan soldiers and an Afghan interpreter.

US forces later tracked the insurgents responsible and called in an air strike with an F-16 fighter jet.

In the aftermath of the attack, the military faced questions about the use of the bulkier, slower-moving Chinooks and the large number of special operations forces assigned to the operation.

But the US commander in Afghanistan, General John Allen, has said he was "comfortable" with the decisions made in the operation and that the Chinooks would still be used in night raids by special forces.