US-led aircraft have dropped nearly 5,000 bombs against Islamic State of Iraq and the Levant (ISIL) targets in Iraq and Syria, as Washington said it was reviewing several incidents in which civilians may have been killed.

According to statistics released by the Pentagon on Wednesday, the US and its allies dropped 4,775 munitions and carried out 1,676 strikes against the armed group.

More than 3,000 targets were either damaged or destroyed, it said.

At least 58 tanks, 184 Humvee armored vehicles, 303 pickup trucks, 26 armored vehicles and 394 other vehicles were struck.

Pentagon spokesman, Steven Warren, said he was "confident the destruction level is high".

American commanders said the raids had halted ISIL's advance in Iraq, but it remained unclear how many tanks or other vehicles the group still has in its arsenal.

Most of the vehicles, many of them US made, had been seized by ISIL fighters from the retreating Iraqi army.

The figures come a day after US officials said for the first time they were investigating at least two incidents in which civilians may have been killed in air strikes in Iraq and Syria, with three other cases being assessed for possible probes.

The current probes involved one case that occurred as recently as December 26, defence officials told the AFP news agency.

The two formal investigations currently under way "are the direct result of our own internal review process and not the result of allegations received from outside of the Department of Defence," Major Curtis Kellogg, a spokesman for US Central Command said.

The Syrian Observatory for Human Rights, a group that monitors the fighting in Syria, said in October that

at least 32 civilians were killed in the first month of coalition strikes.