Secretary of Defense James Mattis speaks during a press briefing at the Pentagon August 28, 2018 in Arlington, Virginia.

"We have worked with Congress and the White House to define the evolving space problem that we confront. Now we are implementing the National Defense Authorization Act and its provision for a unified space command in line with the president's vision for a needed Space Force," Mattis said.

In his opening remarks, Mattis said the U.S. cannot afford "complacency in any domain" and recognized cyberspace and outer space as war-fighting domains on par with land, air and sea.

"We have not done the cost estimates; that's underway right now. We've already commenced the effort, but I don't want to give you an off-the-cuff number," Defense Secretary James Mattis said Tuesday in his first press conference since the U.S.-led April 13 missile strikes against the Syrian government.

The Pentagon does not yet have a cost estimate for President Donald Trump 's requested Space Force, a sixth and separate military branch dedicated to countering the looming threats in outer space.

He added that the Pentagon looks to Congress for the necessary legislation in creating the sixth branch.

Last month, during a discussion at the Brookings Institution, Deborah Lee James, 23rd secretary of the Air Force and principal defense space advisor for the Obama administration, warned that a Space Force would worsen problems plaguing the U.S. military's space efforts.

""It is a virtual certainty that it will be a huge undertaking that will consume a lot of time, effort, thinking," James said.

When asked how expensive a new service branch will be, James said it would cost more than what the Pentagon estimates.

"I do not know how much it would cost to set up a separate military service, but if anyone thinks you're going to do it on the cheap I will tell you that I've never seen anything like this done on the cheap," she said. "So, No. 1, it will cost more than what they predict."

Read more: What Trump's Space Force might look like – and when it would be ready

Trump's sprint to develop a Space Force became apparent during an emphatic June 18 speech to the National Space Council. He announced his wish for the Pentagon to create a new sister service branch to stand alongside the Army, Air Force, Navy, Marine Corps and Coast Guard.

"I am hereby directing the Department of Defense and Pentagon to immediately begin the process necessary to establish a space force as the sixth branch of the armed forces," Trump said before asking Marine Corps Gen. Joseph Dunford, chairman of the Joint Chiefs, to see the directive through.

"Our destiny beyond the Earth is not only a matter of national identity but a matter of national security," Trump said.

Shortly thereafter, Vice President Mike Pence announced at the Pentagon a detailed plan for establishing a Space Force.

"The Space Force will not be built from scratch," Pence said during a speech before members of the Pentagon. "This is a critical step towards establishing the Space Force as the sixth branch of our armed forces."

Read more: Pence unveils plan to create Space Force by 2020

The military has not added a new service since the Air Force more than 70 years ago.

Trump first floated the Space Force idea in March as a part of his national security strategy.

"We have the Air Force, we'll have the Space Force," Trump told an audience of service members at the Marine Corps Air Station Miramar.