Al-Qaida's leadership has assigned cells of engineers to find ways to shoot down, jam or remotely hijack U.S. drones, hoping to exploit the technological vulnerabilities of a weapons system that has inflicted huge losses against the terrorist network, according to top-secret U.S. intelligence documents.

Although there is no evidence that al-Qaida has forced a drone crash or successfully interfered with flight operations, U.S. intelligence officials have closely tracked the group's persistent efforts to develop a counterdrone strategy since 2010, the documents show.

Al-Qaida commanders are hoping a technological breakthrough could curb the U.S. drone campaign, which has killed an estimated 3,000 people over the past decade. The airstrikes have forced al-Qaida operatives and other militants to take extreme measures to limit their movements in Pakistan, Afghanistan, Yemen, Somalia and other places. But the drone attacks have also taken a heavy toll on civilians, generating a bitter popular backlash to U.S. policy toward those countries.