Mounting anger over the conduct of foreign troops had contributed to the convergence of the groups that had previously supported the international engagement in the country, according to the study published by the independent Afghan Analysis Network.
Should this trend continue, the insurgency had the potential to transgress ethnic and religious divides to extend to a nationalistic level.
In addition to the "deepening sense of occupation" by foreign troops, other factors were contributing to the appeal of the Taliban for non-Pashtuns, the study's author Thomas Ruttig said.
"Undercurrents of anti-Westernism" and "a surge in international Muslim solidarity" were listed among these factors.
The study, which found that past attempts at "reconciliation" with insurgents had failed "because of the wrong assumptions", recommended a fresh "reconciliation strategy" that encompasses more than talks with the various insurgent groups.