The United States will offer a $15,000 annual bonus for drone pilots, the Air Force said -- part of a drive to address what it admitted was a "critical shortage" of manpower.

Drones are an essential part of Washington's arsenal in the battle on extremists abroad, responsible for taking out several-high profile jihadist targets from Yemen to the lawless border between Afghanistan and Pakistan.

But US officials have been increasingly concerned about the number of pilots of remotely piloted aircraft (RPA) at their disposal, and the physical and mental demands on those left in the job.

As well as offering recruits a retention bonus of $15,000 per year for five- and nine-year stints starting in 2016, the US Air Force says it is investing $100 million to buy more ground control stations, simulators and contract instructors, part of a recruitment and retention drive.

It will also include 80 newly trained pilots over the next 12 months.

"In a complex global environment, RPA pilots will always be in demand," said Secretary of the Air Force Deborah Lee James.

"Remarkable airmen have ensured the success of the (MQ-1 Predator and MQ-9 Reaper drone) programs.

"We now face a situation where if we don't direct additional resources appropriately, it creates unacceptable risk."