Defense Secretary Lloyd Austin wrote a letter strongly criticizing an ongoing hold on senior military promotions and nominations in the Senate led by Republican lawmakers, saying it would create a "perilous precedent" for the military and impose "unconscionable" burdens on military families.

The letter, sent on May 5 in response to a query from Democratic Sen. Elizabeth Warren, addressed the effort spearheaded by Alabama Republican Sen. Tommy Tuberville, calling it "irresponsible" and "unprecedented." Austin also said it undermines the US military's credibility abroad, and will impose "new and unnecessary risk on US warfighters across multiple theaters of operations."

"Ultimately, the breakdown of the normal flow of leadership across the Department's carefully cultivated promotion and transition system will breed uncertainty and confusion across the US military," Austin said. "This protracted hold means uncertainty for our service members and their families and rising disquiet from our allies and partners, at a moment when our competitors and adversaries are watching."

Tuberville has maintained his hold on general and flag officer nominations in protest of new reproductive health policies instituted throughout the US military, which among other things provide leave and travel allowances for troops or their families who must travel to receive an abortion.

The policies also extend the timeline in which service members must tell their commanders they are pregnant, and provides up to 21 days of leave for troops seeking in vitro fertilization (IVF) and intrauterine insemination (IUI).

"The efforts taken by the Department today will not only ensure that service members and their families are afforded time and flexibility to make private health care decisions, but will also ensure service members are able to access non-covered reproductive health care regardless of where they are stationed," a Pentagon news release outlining the new policies said.

Tuberville, however, has argued that federal funding cannot be used to perform abortions except in cases of rape, incest, or when the mother's life is at risk. He has received support from roughly a dozen other Republicans in the Senate and House, who called the Pentagon's policies "illegal," "atrocious," and "immoral."

The US military still does not perform abortions except for in covered cases, which applies only to the three aforementioned situations.

Defense officials have warned that the hold on nominations would negatively impact military readiness and throw a wrench into an otherwise routine system of promotions and moving families to new assignments.

There are currently 180 nominations pending in the Senate, including the commanders of the Navy's 7th and 5th Fleets, which cover the Indo-Pacific and Naval Forces Central Command, respectively. It also includes the military representative to NATO.

In his letter to Warren, Austin said 64 three- and four-star positions are expected to rotate out to a new flag or general officer within the next 120 days, including some of the most senior military commanders -- the chief of staff of the Army, chief of Naval Operations, commandant and assistant commandant of the Marine Corps, commander of US Northern Command, and director of the National Security Agency and commander of United States Cyber Command.

In the coming months, those positions up for nomination will also include the chairman of the Joint Chiefs of Staff -- the military's most senior uniformed leader -- and the vice chiefs of staff of the Army and Air Force.

"Without these leaders in place, the U.S. military will incur an unnecessary and unprecedented degree of risk at a moment when our adversaries may seek to test our resolve," Austin warned.

The hold will make it "harder for the United States to fulfill its global leadership responsibilities" around the globe, he goes on to say, and potentially force "out units to operate with less experienced decision makers in charge."

It could also have a trickle-down effect on other military officers, whose promotions are "possible only with the retirement of others," Austin added.

Without the regular promotion and retirement of officers at the top, Austin said the military could experience delayed or canceled permanent change of station moves, which is already an often-hectic process for service members and their families who are moving to new installations for new assignments.

The Senate approves thousands of civilian and military nominations every year, typically through unanimous consent. But with Tuberville's block on the process, it could take months to go through each military nomination one at a time, which a Democratic Senate aide previously told CNN could take a year to complete.

But Austin warned in his letter to Warren that the nominations need to be approved as quickly as possible.

"This indefinite hold harms America's national security and hinders the Pentagon's normal operations. The United States military relies on the deep experience and strategic expertise of our senior military leaders," he said. "The longer that this hold persists, the greater the risk the US military runs in every theater, every domain, and every service."