Defense Secretary Lloyd Austin will seek to mandate coronavirus vaccination for all U.S. service members by mid-September and could begin requiring inoculation even sooner if a vaccine receives full approval from the Food and Drug Administration, the Pentagon said Monday.

The effort is an acknowledgment that rising infection rates across the country pose a particular threat to military readiness, and it follows a months-long campaign by senior defense officials to cajole the nation’s 1.3 million active duty service members to get vaccinated voluntarily. The Biden administration, alarmed by continued spread of the virus’s delta variant and vaccination rates that remain low in several pockets of the country, has directed agencies throughout the federal government to devise such plans.

About 65 percent of the active-duty military is fully inoculated, according to Pentagon data, but rates vary significantly by service as misinformation about the vaccines’ safety has fueled reluctance among a large segment of the workforce. The Navy, which leads the way, says that about 75 percent of its people are fully inoculated. On the low end, the Marine Corps stands at about 59 percent.

“The intervening few weeks will be spent preparing for this transition,” Austin said in a single-page memo circulated to the Defense Department workforce. “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.”

The message stopped short of establishing a deadline by which all personnel must be vaccinated and did not set a time frame for the individual services to provide plans to Austin. It comes about a week after President Biden announced that he will require federal employees to get vaccinated or undergo repeated testing.

Biden, who must approve Austin’s request, quickly praised the decision as millions of Americans remain resistant to vaccines, and governments and employers increasingly turn to mandates. Overall, only about half of the U.S. population is fully vaccinated, although rates have begun to rise amid the most recent surge in hospitalizations.

“We cannot let up in the fight against COVID-19, especially with the Delta variant spreading rapidly through unvaccinated populations,” the president said in the statement. “We are still on a wartime footing, and every American who is eligible should take immediate steps to get vaccinated right away.”

In a separate message, the Pentagon’s top uniformed officer, Army Gen. Mark A. Milley, said that medical professionals in the Defense Department have recommended vaccination as a “necessary step to sustain our readiness and protect our force, our coworkers, our families, and our communities.”

“Mandating vaccines in the military is not new,” said Milley, chairman of the Joint Chiefs of Staff. “Since the first days of basic training and throughout our service, we’ve received multiple vaccines. We have proven processes with trusted and skilled medical professionals.”

Senior military leaders have wrestled with coronavirus outbreaks throughout the pandemic, including one that crippled the aircraft carrier USS Theodore Roosevelt while at sea last year. The ship spent weeks in port in Guam, with more than 1,200 sailors of about 4,800 aboard testing positive for the virus and one dying.

Overall, 28 service members have died as a result of the pandemic, according to Pentagon statistics. An additional 254 civilians working for the Defense Department, 16 dependents and 87 defense contractors also have died.

Coronavirus vaccines are presently approved with an emergency-use authorization as the FDA continues to test them. Senior Pentagon officials have stopped short of requiring vaccination in the interim, while participating in public-awareness campaigns urging service members to get vaccinated.

John Kirby, a Pentagon spokesman, said that if the FDA grants full approval for a coronavirus vaccine before September, Austin will have authority to require personnel to get that version of the vaccine immediately, without Biden’s approval. Media reports indicate that Pfizer’s version of the vaccine could receive full approval by late August, he noted.

Once a coronavirus vaccine is required for military personnel, it will likely be tracked as a part of each service member’s medical record along with other compulsory vaccinations. Kirby declined to speculate on what will happen to service members who refuse to take it once it is required, but in the past the military has removed such people from service through administrative separation.

In coming days, officials who lead the individual service branches will be required to prepare a plan for Austin detailing how they will carry out the mandate. Kirby said that the memo released Monday serves as a “warning order” for the services and individual troops to get ready.

In his memo, Austin urged service members not to wait for the mandate, emphasizing that the vaccines approved for emergency use in the United States have proven “safe and highly effective.”

“They will protect you and your family,” Austin wrote. “They will protect your unit, your ship, and your co-workers. And they will ensure we remain the most lethal and ready force in the world.”