The tally of civilians killed by U.S.-led air strikes on an Afghan village in August has more than quadrupled, according to the commander of U.S. forces in the Middle East.

The attack on a suspected Taliban compound in Azizabad killed 33 civilians, Lt. Gen. Martin Dempsey said in a statement summarizing the findings of a U.S. Central Command investigation into the incident.

"We are deeply saddened at the loss of innocent life in Azizabad," Dempsey said Wednesday.

An earlier U.S. investigation into the attack declared that five to seven civilians were killed in the Aug. 22 attack, as well as up to 35 Taliban fighters.

That investigation followed allegations from the Afghan government — supported by a UN preliminary report — that 90 civilians had died.

The earlier U.S. report, issued in September, left open the possibility that evidence could emerge to prove that more people died in Azizabad, which is in western Herat province.

It also said the air strikes were in response to militant ambushes of coalition troops in Herat, the U.S. military has said. U.S. and Afghan forces reportedly began taking fire from militants as the forces approached Azizabad in the early hours of Aug. 22.

The incoming fire "justified use of well-aimed small-arms fire and close air support to defend the combined force," the September report said.

Dempsey reiterated that position Wednesday, saying the U.S. forces had acted in self-defence and on credible intelligence, within the rules of engagement.

The rising number of civilian deaths in Afghanistan has caused increased tension between Afghan President Hamid Karzai and Western forces fighting in the country.

The number of Afghan civilians killed in insurgent attacks and air strikes by NATO and U.S. forces has risen 40 per cent in the past year, the United Nations said in a report released in September.

Among the 577 civilians killed by U.S.-led Western forces or the Afghan National Army, more than two-thirds were killed by air strikes, the UN report said. In the same period last year, pro-government forces were responsible for 477 civilian deaths.

After the bombing in Azizabad, the Afghan government announced it would review its "status of force" agreement with the U.S. and NATO and review whether to demand an end to air strikes and operations in Afghan villages.

With files from the Associated Press