The world is less peaceful than a decade ago, mostly due to conflict in the Middle East and Africa that is costing the global economy trillions of dollars, an international index showed on Wednesday.

"There's been a gradual decline in peacefulness over the last decade," said Steve Killelea, head of the Institute for Economics and Peace (IEP), which is headquartered in Australia.

"The reason for this slow, gradual decline in peacefulness really comes back to the conflicts in the Middle East and North Africa, and the spillover effects into other areas," Killelea said in a phone interview.