File Photo: Afghan army soldiers are deployed to Achin district of Nangarhar province, Afghanistan, after the U.S. military dropped a GBU-43 or Massive Ordnance Air Blast (MOAB) bomb on an IS cave complex in Achin district, on April 14, 2017. (Xinhua/Rahman Safi)

WASHINGTON, Nov. 9 (Xinhua) -- The United States may have spent a whopping 5.6 trillion U.S. dollars on regional wars since the Sept.11 terror attack in 2001, more than triple official figures, according to one study released Thursday.

The study by Brown University as a part of a cost of wars project, said U.S. combat missions in Iraq, Afghanistan, Syria and Pakistan have cost the average taxpayer 23,386 dollars over the past 16 years.

The Pentagon said in a July report that the United States has spent 1.52 trillion dollars, but researchers said the figure was incomplete without taking into account costs incurred by the Departments of State, Veteran Affairs and Homeland Security, as well as the interest on federal borrowings.

"The American public should know what the true costs of these choices are and what lost opportunities they represent," said Catherine Lutz, project co-director and a Brown University professor.

"Given that the current administration has announced more years of war in Afghanistan and elsewhere, this total will only grow," she said.

Neta Crawford, another co-director of the project, said the Brown University report has also left some numbers out.

"For example, there are substantial costs of war to state and local governments that are not subsidized by the federal government, most significantly, perhaps, the costs of caring for the veterans of these wars," said Crawford.

"A full accounting of any war's burdens cannot be placed in columns on a ledger," Crawford said.