ISLAMABAD, Pakistan – 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 nine-year-long conflict.

They also highlight the importance of recently opened supply routes into landlocked Afghanistan through central Asian states to its north. Those routes are safer, but the Pakistani lines from the Arabian seaport of Karachi north to Kabul and Kandahar in Afghanistan are cheaper and account for most of NATO’s non-lethal supplies.

Pakistan shut down the Torkham border crossing — the most important NATO supply into Afghanistan — on Thursday in apparent protest of a NATO helicopter attack that killed three Pakistani soldiers on the frontier. It was the third such incursion into Pakistan in less than a week.

The other NATO supply line through Pakistan remained open — the Chaman crossing in Baluchistan, where it seemed likely the tankers were heading.

A lengthy closure of Torkham would place intense strain on the U.S.-Pakistani relationship and hurt the Afghan war effort. But a long shutdown continued to be seen as unlikely.

Senior U.S. officials acknowledged high tension between the two capitals that crested with the border closure.

On the Pakistani side, the incursions into Pakistan by U.S. forces fighting in Afghanistan provoked an unusually strong government condemnation. On the U.S. side, publication of a video that may show Pakistani military officers summarily executing insurgents threatened to undermine public and congressional support for U.S. aid.

Keeping the crossing closed briefly could also provide Pakistan’s fragile civilian government a visible symbol that it is willing to stand up to its U.S. backers.