US-led air strikes against Islamic State militants in Iraq and Syria have damaged or destroyed 3,222 targets since August, including 58 tanks, 184 Humvees, 673 fighting positions and 980 buildings or barracks, the Pentagon said on Wednesday.

Army Colonel Steve Warren, a Pentagon spokesman, said he was not sure how many targets had been damaged rather than destroyed, but said: "I'm confident that the destruction level is high. Our strikes are extraordinarily accurate."

Release of the target damage list came a day after US defence officials confirmed they are looking into reports of civilian casualties caused by the strikes in Iraq and Syria and are conducting a deeper investigation of two cases involving fewer than five deaths.

As of 11 pm on Tuesday (0400 GMT Wednesday), US and coalition air forces had conducted a total of 1,676 air strikes against Islamic State militants in Iraq and Syria since August 8 and had used some 4,775 munitions, a Pentagon spokeswoman said.

Warren said US and coalition forces had hit 3,222 targets in the air strikes but he declined to say what percentage of Islamic State equipment was destroyed.

"In order for us to do that we would have to release to you the exact number of tanks we believe the enemy has, the exact number of Humvees we believe the enemy has," he said. "We don't want our enemy to know how much we know about them."

Countries whose forces have participated in the strikes in Iraq are: Australia, Belgium, Britain, Canada, Denmark, France and the Netherlands. Those participating in strikes in Syria are Bahrain, Jordan, Saudi Arabia and the United Arab Emirates.

The targets damaged or destroyed included tanks, boats, Humvees, technical vehicles, artillery, buildings and oil infrastructure sites. No aircraft were on the list, Warren said.

He said the Humvees were US vehicles supplied to the Iraqi military and captured by militants, as were some of the armoured personnel vehicles. It was unclear whether any of the tanks were US-made.

Warren said some vehicles had been hit while parked, while others were struck while engaged in combat. The boats were used by Islamic State to ferry supplies across the Euphrates River, he said.