Researchers say a drone aircraft being tested by the U.S. Navy that could conduct a combat mission without human involvement raises troubling ethical questions.

The drone, designed to land on the deck of an aircraft carrier, operates not only with no pilot in the cockpit but with no pilot at all, raising the specter of a pre-programmed semi-independently operating machine capable of wreaking mayhem on its own.

While humans would program the autonomous drone's flight plan and could override its decisions, many find the concept of a heavily armed aircraft operating without direct human control worrisome.

"Lethal actions should have a clear chain of accountability," Noel Sharkey, a computer scientist and robotics expert, told the Los Angeles Times. "This is difficult with a robot weapon.

"The robot cannot be held accountable. So is it the commander who used it? The politician who authorized it? The military's acquisition process? The manufacturer, for faulty equipment?"

A U.S. Air Force report on the future of drones said it's only a matter of time before the machines have the capability to make life-or-death decisions over the battlefield.

However, officials would still monitor how the drones were being deployed, it said.

"Increasingly humans will no longer be 'in the loop' but rather 'on the loop' -- monitoring the execution of certain decisions," the report said. "Authorizing a machine to make lethal combat decisions is contingent upon political and military leaders resolving legal and ethical questions."