Senator Tommy Tuberville, a Republican, is stalling the approval of more than 300 US military nominees to protest Defense Department efforts to assist troops who must travel to receive reproductive health care that is unavailable where they are stationed.

The officers selected to head the Army, Navy and Marine Corps are among the nominees Tuberville is keeping from being quickly approved by the Senate via unanimous consent.

Lawmakers can still vote on nominations individually, but that much more time-consuming process has not been used so far.

"Because of this blanket hold, starting today, for the first time in the history of the Department of Defense, three of our military services are operating without Senate-confirmed leaders," US Defense Secretary Lloyd Austin said at a ceremony marking Admiral Mike Gilday's departure from office.

"This is unprecedented. It is unnecessary. And it is unsafe," Austin said

Admiral Lisa Franchetti, who is currently vice chief of naval operations and has been nominated to lead the service, will perform the top job in an acting capacity in addition to her current position.

The US Supreme Court in June 2022 struck down the nationwide right to abortion, meaning troops stationed in places that restricted or banned the procedure must now take leave and travel to areas where it is legal to obtain one.

In response, Austin directed the Defense Department to develop policies to allow service members to take administrative absences to receive "non-covered reproductive health care," and to establish travel allowances to help them cover costs.

Tuberville, whose home state of Alabama has instituted a near-total ban on abortions, insists those efforts are illegal and has vowed to delay the approval of senior officers as well Defense Department civilian officials until it is reversed.

The US Marine Corps commandant stepped down on July 10, and the general nominated to replace him has been performing two jobs -- vice commandant and head of the service -- for more than a month.

The chief of staff of the Army left office on August 4, and his deputy has since then been in the same situation as his counterpart in the Marine Corps.

The problem is set to worsen further with the upcoming departure of Chairman of the Joint Chiefs of Staff General Mark Milley, the president's top military advisor.