ARLINGTON, VIRGINIA - JULY 21:  U.S. Secretary of Defense Lloyd Austin (L) and Chairman of Joint Chiefs of Staff Gen. Mark Milley (R) participate in a news briefing at the Pentagon July 21, 2021 in Arlington, Virginia.
Washington CNN  — 

Secretary of Defense Lloyd Austin is moving to have all active duty members of the US military vaccinated against the Covid-19 virus, he said in a memo released Monday.

“I want you to know that I will seek the President’s approval to make the vaccines mandatory no later than mid-September, or immediately upon the U.S. Food and Drug Administration (FDA) licensure, whichever comes first,” the memo states. “By way of expectation, public reporting suggests the Pfizer-BioNTech vaccine could achieve full FDA license early next month.”

Chairman of the Joint Chiefs Gen. Mark Milley also communicated a related message to the troops, saying in a memo that the “health and readiness of our force is critical to America’s defense.”

Austin’s memo noted that he was asked by President Joe Biden “to consider how and when we might add the coronavirus disease 2019 (COVID-19) vaccines to the list of those required for all Service members.”

But details about the plan for implementing the mandate remain unclear.

“The intervening few weeks will be spent preparing for this transition. I have every confidence that Service leadership and your commanders will implement this new vaccination program with professionalism, skill, and compassion. We will have more to say about this as implementation plans are fully developed,” Austin wrote.

The Associated Press first reported the move.

Biden said in a statement of his own Monday that he supports Austin’s message to the force and plans to make the Covid-19 vaccine mandatory for all active duty military members.

“I strongly support Secretary Austin’s message to the Force today on the Department of Defense’s plan to add the COVID-19 vaccine to the list of required vaccinations for our service members not later than mid-September,” Biden wrote.

“Secretary Austin and I share an unshakable commitment to making sure our troops have every tool they need to do their jobs as safely as possible,” Biden continued. “Being vaccinated will enable our service members to stay healthy, to better protect their families, and to ensure that our force is ready to operate anywhere in the world.”

The President was briefed Friday by Austin and Milley on the options for mandating Covid-19 vaccines within the Department of Defense.

While it was clear this was the path Biden wanted to pursue, the process is complicated, given the President must sign a waiver ordering it within the military if the vaccine does not have full approval from the FDA, according to a senior administration official.

Monday’s memo from the Pentagon says vaccinations will be required starting in mid-September. If no Covid-19 vaccine has full approval by then, Biden is expected to sign a waiver that says members of the military do not have the right to refuse the coronavirus vaccine.

CNN reported last week that Austin was expected to seek authorization to make Covid-19 vaccines mandatory for all active duty troops following Biden’s directive that the military examine how and when it could make that happen.

Austin’s “inclination is towards making the COVID-19 vaccine mandatory” for active duty troops, a defense official said at the time.

Biden announced on July 29 that he was asking the Defense Department to “look into how and when” it will add the Covid-19 vaccine to the list of mandatory military vaccinations. Biden specifically said he knew that Austin is “open to it.”

Since then, the Joint Chiefs have met to discuss how a mandatory vaccination plan might work. Austin has also been consulting with military medical authorities, the defense official said. Austin’s current view is to “seek authorization to make it mandatory.”

The Pentagon previously indicated it was likely to wait for FDA approval before making inoculation against Covid-19 mandatory, but Biden’s push for greater vaccination levels created pressure to move more quickly and will likely lead the Pentagon to request a presidential waiver.

A Justice Department memo dated July 6 said that “because DOD has informed us that it understandably does not want to convey inaccurate or confusing information to service members—that is, telling them that they have the ‘option’ to refuse the COVID-19 vaccine if they effectively lack such an option because of a military order—DOD should seek a presidential waiver before it imposes a vaccination requirement.”

This story has been updated with additional developments Monday.

CNN’s John Harwood contributed to this report.