The US military is investigating whether it was responsible for the deaths of nearly 300 Syrian and Iraqi civilians in three different sets of airstrikes this month, a media report said.

Civilian casualties have been alleged in all three instances, but each situation is different and complex, CNN quoted a US defence official as saying on Friday.

So far, there is no indication of a breakdown in US military procedures governing airstrikes, the official said, and the US is not contemplating a pause in military operations.

The most severe incident involves western Mosul in Iraq.

The US military is trying to determine if sometime between March 17 and March 23, bombs dropped in the neighbourhoods of al Jadidah, al Amel and al Yarmouk by American warplanes resulted in the deaths of over 200 civilians.

The chairman of Nineveh Provincial Council in Iraq, Bashar al Kiki, told CNN: "Most of (those) killed are civilians, among them children and women."

The Iraqi official demanded an end to military operations in the area until civilians' safety can be guaranteed.

The incidents military officials are looking into are based largely on local reports and social media accounts of the strikes.

While in Syria, the US military has begun a formal investigation into a March 16 airstrike, where local reports said a mosque was struck and more than 40 people died.

For days the Pentagon said there were no civilian casualties in the March 16 incident, even as numerous social media reports showed images of bodies being carried out of the rubble.

However, the US has not ruled out the possibility that the Islamic State (IS) terror group was using civilians as human shields, but the defence official told CNN that there was an urgency to find out if Washington was responsible.

The Central Command was also reviewing an airstrike against a school building on Wednesday near Raqqa, Syria.

Local activists have said an airstrike may have killed more than 30 civilians seeking shelter there. The US was conducting strikes in the area, the defence official added.