About half a million people have died violently in Iraq, Afghanistan and Pakistan due to the US "war on terror" that was launched following the September 11 attacks in 2001, according to a study released on Thursday.

The report by Brown University's Watson Institute for International and Public Affairs put the death toll at between 480 000 and 507 000 people - but said the actual number is likely higher.

The new toll "is a more than 110 000 increase over the last count, issued just two years ago in August 2016," Brown said in a statement.

"Though the war on terror is often overlooked by the American public, press and lawmakers, the increased body count signals that, far from diminishing, this war remains intense."

The death toll includes insurgents, local police and security forces, civilians and US and allied troops.

The report's author, Neta Crawford, said many of those reported by US and local forces as militants may actually have been civilians.

"We may never know the total direct death toll in these wars," Crawford wrote.

"For example, tens of thousands of civilians may have died in retaking Mosul and other cities from ISIS but their bodies have likely not been recovered."

The report states that between 182 272 and 204 575 civilians have been killed in Iraq, 38 480 in Afghanistan, and 23 372 in Pakistan.

Nearly 7000 US troops have been killed in Iraq and Afghanistan.

The tally does not include all people who have died indirectly as a result of war, including through a loss of infrastructure or disease.