Austin said he will issue an order making vaccines mandatory across all the services by September 15, or earlier if the Pfizer or other vaccines get full approval from the US Food and Drug Administration before then, the memo said.

President Joe Biden released a statement "strongly" supporting the decision, saying: "These vaccines save lives."

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

Over half of the 2.5 million active duty troops and national guard have been vaccinated, according to Pentagon statistics.

That 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.