Pakistan blocked a vital supply route for U.S. and NATO troops in Afghanistan on Thursday in apparent retaliation for an alleged cross-border helicopter strike by the coalition that killed three Pakistani frontier soldiers.

The blockade appeared to be a major escalation in tensions between Pakistan and the United States.

A permanent stoppage of supply trucks would place massive strains on the relationship between the two countries and hurt the Afghan war effort. Even a short halt is a reminder of the leverage Pakistan has over the United States at a crucial time in the 9-year-old war.