Defense Secretary Lloyd Austin said in a memo he would ask President Joe Biden to approve the move in about five weeks, even if none of the current vaccines gains full approval from the US Food and Drug Administration.

But the order could come earlier if the Pfizer vaccine gets approval before then. Officials say they think Pfizer could earn full FDA approval by the beginning of September.

Because the Covid vaccines have only had emergency approval, the US military has until now not forced troops to have them, as it does for other inoculations.

Doing so would have opened the door to legal challenges, unless Biden issued a waiver to allow it.

If the Pfizer or another vaccine does not get full FDA approval by mid-September, Austin said he would go ahead and seek Biden's approval.

Biden released a statement "strongly" supporting the decision.

"These vaccines save lives," he said.

"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."

Defense Department spokesman John Kirby said that the Pentagon expects to have the hundreds of thousands of doses needed to be stockpiled and that individual services are currently developing plans for the forced vaccinations.

Currently 73 percent of 1.4 million active duty forces have had at least one shot, but if another 1.1 million in reserve troops are added in, the rate goes down to 56 percent.

That low level has left US officials concerned over the potential for the virus to hurt military readiness.

Austin said in the memo that before the vaccination order is made, the military would "also be keeping a close eye on infection rates -- which are on the rise now due to the Delta variant -- and the impact these rates might have on our readiness."

"I will not hesitate to act sooner or recommend a different course to the president if I feel the need to do so," he said.

US service members currently are mandated to have 17 vaccines, all of which are FDA-approved. Making the Covid-19 vaccine compulsory also raised threats of intervention from some members of Congress.

But Representative Mike Rogers, the senior Republican on the House Armed Services Committee, said the vaccines were important for military readiness.

"We must not allow Covid-19 to be a hindrance on our force," he said.

"Vaccines protect our men and women, many of whom live in cramped and crowded conditions, from the spread of disease while at home or deployed across the globe."