According to the report by the institute Chicago Project on Security & Terrorism (CPOST), Iraq and Afghanistan accounted for over 60 percent of the world's suicide bombings in 2014.
"That's particularly worrisome from a U.S. policy perspective, because the United States wants to withdraw militarily from Afghanistan, but there are no signs that the campaign by the Afghan Taliban is letting up," CPOST research director and author of the report Keven Ruby said, as quoted by the University of Chicago news portal.
In 2011, at the height of US involvement in the anti-Taliban confrontation, there were 101,000 US troops in the country.
Under the timetable outlined by US President Barack Obama, the number of US troops is expected to be cut to about 5,000 by the end of 2015, and by the end of 2016 there will be only a vestigial force to protect the US embassy in Kabul.
According to the Suicide Index report, a total of 492 suicide attacks that killed over 4,300 civilians were registered in 2014. Iraq, partly occupied by the Islamic State militant group, along with swathes of neighboring Syria, saw a surge in suicide attacks last year due to the extremists' activity.