The Pakistani Taliban continues to target NATO fuel trucks that support U.S. troops in Afghanistan.

Militants attacked 20 tankers, Monday, killing four people and injuring seven others.

Hours later, gunman attacked and burned two other NATO supply trucks in southwest Pakistan. One driver was killed.

There have been four NATO convoy attacks since Pakistan closed a supply crossing into Afghanistan.

Traffic has since been backing up at various points along the supply route from the southern port city of Karachi to Torkham -- where several trucks remain stranded and vulnerable to attack.