WASHINGTON (AP) — President Donald Trump is moving closer toward his goal of creating a Space Force, just not as the separate military department he envisioned.

The Space Force instead will begin as part of the Air Force - similar to how the Marine Corps is part of the Navy - but could become its own separate department in the future. That's according to senior administration officials who briefed journalists on a directive Trump planned to sign Tuesday establishing the Space Force.

Trump says it's needed to ensure U.S. dominance in space. Some lawmakers have expressed concerns about the need and potential cost.

Cost details will be included in the 2020 budget proposal Trump sends Congress next month.

Administration officials spoke on condition of anonymity under grounds rules set by the White House.