President Donald Trump says the US should create a military “Space Force” to defend his country’s interests outside the earth’s atmosphere.

“Space is a war-fighting domain, just like the land, air, and sea,” Mr Trump told marines at a Californian air base on Wednesday (Australian time).

“We may even … develop another one, Space Force. We have the Air Force, we’ll have the Space Force.”

Mr Trump, who earlier fired secretary of state Rex Tillerson, said he’d originally coined the term “Space Force” as a joke while discussing US government spending, and private investment, in space.

“I said, ‘maybe we need a new force, we’ll call it the Space Force’, and I was not really serious. Then I said, ‘what a great idea’, maybe we’ll have to do that,” he told the servicemen.

“So think of that, Space Force, because we are spending a lot and we have a lot of private money coming in, tremendous.

“From the very beginning, many of our astronauts have been soldiers and airmen, coast guard men and marines. And our service members will be vital to ensuring America continues to lead the way into the stars.”

President Trump addresses Marines in California:

"We're doing a tremendous amount of work in space, and I said, maybe we need a new force. We'll call it the 'Space Force.'" pic.twitter.com/Ed3KiOKVSk

— NBC News (@NBCNews) March 13, 2018

The President’s remarks come after a Republican plan to create a “Space Corps” as part of the Air Force was dropped from the National Defence Authorisation Act in November.

The proposal would have set up a Space Corps in the mould of the Marine Corps, which is a separate military branch housed within the Navy.

Republican congressman Mike Rogers, who proposed the idea, argued the Air Force was prioritising its fighter jets over space, and a dedicated service was needed to stay ahead of China and Russia in what was described as the next frontier of warfare.