The U.S. Marine Corps has extended indefinitely its use of two unmanned K-MAX heavy lift helicopters in Afghanistan, a Navy spokeswoman said, noting the aircraft kept re-supply trucks off the road and reduced the threat of roadside bombs.

The Marine Corps sent the new autonomous helicopters, which were developed by Lockheed Martin Corp and privately-held Kaman Aerospace, to Afghanistan in November 2011 for an initial, limited deployment, but has extended their use several times.

Now, Naval Air Systems Command has decided to continue using the aircraft indefinitely, said Jamie Cosgrove, a spokeswoman for the Navy command, which also oversees Marine Corps aircraft.

She said there were no current plans to buy more K-MAX helicopters, but the two aircraft in Afghanistan would remain there “until otherwise directed.”

Dan Schultz, vice president of Lockheed’s Aviation Systems business, said the two K-MAX helicopters had flown more than 1,000 missions in Afghanistan and hauled more than 3 million pounds of cargo that would have otherwise been transported by trucks, which are vulnerable to roadside bomb attacks.

K-MAX was “saving lives by reducing Marines’ exposure to improvised explosive devices on cargo convoys,” he said in a statement to Reuters.

First developed to meet a “joint urgent operational need statement” issued by the U.S. military in 2010, K-MAX can fly autonomously over large distances, and at higher altitudes with a larger payload than any other rotary wing unmanned aerial system, according to Lockheed. It can accurately deliver cargo as large as a compact car.

Unlike the popular Predator drones, which are remotely piloted, K-MAX helicopters follow a pre-programmed route using Global Positioning System (GPS) coordinates, and require human intervention only to get started.

The helicopters can also fly during bad weather, at night, and in the rain. Last year Lockheed said the helicopters also required two hours fewer maintenance per flight hour, which makes them cheaper to operate.

Popular Science magazine in November named the unmanned K-MAX helicopter to its “Best of What’s New” list.

Cosgrove said Marine Corps officials were now assessing how to use the helicopters after this deployment.

One senior Marine Corps official said the helicopter was proving useful, but its “niche” in future Marine Corps aviation plans was not yet clear.

U.S. forces are due to withdraw from Afghanistan in 2014.