Assailants launched two separate attacks on tankers carrying fuel for foreign troops in Afghanistan on Friday, showing the vulnerability of NATO supply lines a day after the Pakistani government itself shut one down.

The events stand to complicate a difficult war in Afghanistan, especially if the Torkham border crossing along the fabled Khyber Pass remains closed for long. They are a reminder of the leverage Pakistan has over the United States just as Washington seeks the help of its uncomfortable ally at a crucial point in the 9-year-long conflict.