Faced with greater security challenges around the world, the US military plans to expand drone flights by 50% over the next four years.

Currently, the military runs as many as 65 daily flights. By 2019, this number will have risen to as many as 90 air patrols. That's a significant increase considering that in 2004, there had only been as few as five drone flights per day.

"We've seen a steady demand signal from all of our geographic combatant commanders to have more of this capability," said Navy Captain Jeff Davis, a Defense Department spokesman.

Until recently, most of the drone flights have been operated by the Air Force, leaving its pilots and crews overworked. This prompted its leadership to ask for a reduction in flights.

Defense Secretary Ashton Carter has agreed to bring the number down to 60 Air Force flights, starting this October, on condition that other branches of the military will pick up the slack.

Under the plan, the Army will contribute as many as 16 daily combat air patrols, and the Special Operations Command, as many as four. Civilian contractors will provide as many as 10 surveillance missions per day.

"I'm glad to hear they're taking a more joint approach," retired Air Force three-star general David Deptula told the Wall Street Journal. "That'll be a great help right there."

The plan, which provides for increased capacity for lethal airstrikes, will come in stages. The Wall Street Journal reports that by 2017, the Army will have been running as many as eight missions per day, while the contractors, six flights.

To expand drone capability, flights grounded due to certain restrictions will be rerouted to other locations. For instance, drones that could not fly over one area due to bad weather would be sent elsewhere.

The Pentagon is also planning to use wide-area airborne surveillance pods. With this technology, more cameras can be installed on the drone's belly, increasing the number of surveillance feeds by a factor of ten.

The expanded drone program boosts the firepower and intelligence-gathering capabilities of military commanders, who have been sharing feeds with the Central Intelligence Agency. Using military personnel, the spy agency directs 22 of the Air Force's daily flights.

"It's the combatant commanders, they need more. They're tasked to do our nation's business overseas so they feel that stress on them, and it's not getting better," Air Force Maj. Gen. J.D. Harris, Jr. told ABC News.

The military has been using Predator and Reaper drones to gather intelligence and target terrorists primarily in Iraq, Afghanistan, Pakistan, Somalia, and Yemen.

But in the coming years, top military officials say needs for those unmanned aerial vehicles may shift to Europe and the Pacific. That's because of increasing aggression from Russia and China, which has been flexing its military muscle by building islands in the disputed South China Sea.

One senior defense official told the Wall Street Journal current global hotspots are Ukraine, the South China Sea, Iraq, Syria, and North Africa.

It is not clear how the Pentagon will fund the additional flights. Officials say they may tap into "war funding," an account that funded counter-terror operations in the Middle East.

There are fears the expansion in drone work will cause greater collateral damage. Based on estimates by non-partisan groups at least 3,000 people have been killed in US drone strikes since 2004, many of them civilians.

US officials have said strikes by drones are carried out against top Islamist militants, only if there are no means to capture them. There is no international legal framework regulating their use.

"People need to know the lack of oversight, the lack of accountability that happen," former US drone sensor operator Brandon Bryant told Russian media RT.