WASHINGTON — U.S. military troops may be able to sidestep the Pentagon's entrenched "up or out" promotion system under sweeping new proposals being unveiled Thursday, aimed at keeping high-tech experts or other specialists on the job, according to defense officials.

Defense Secretary Ash Carter is expected to roll out the plans Thursday, marking the third — and most groundbreaking — installment in his campaign to modernize the military's antiquated bureaucracy. The proposals are largely aimed at giving the military services a greater ability to attract or hold on to quality service members and keep them in jobs where they excel.

Carter's plan, hammered out by staff and senior military leaders over recent months, won't abolish the traditional system that forces service members to leave if they don't get promoted within a certain period of time. Instead, the officials said it will give the services the flexibility to bypass those rules for individuals when they feel it's needed.

Several defense officials spoke about the details of the plans on condition of anonymity because they weren't authorized to discuss the issue publicly ahead of the announcement. Military leaders have expressed varying degrees of support for the promotion changes, noting that in some fields — such as fighter pilots or certain combat command positions — the strict advancement system may make more sense.

Many of the proposals will require congressional approval, but there is some general support for giving the military greater flexibility, as long as the historical systems aren't eliminated.

One idea likely to hit opposition on Capitol Hill is Carter's proposal that the Pentagon give department civilians six weeks of paid paternal leave for the birth or adoption of a child. There are currently rules for family leave that apply to the entire federal workforce, and it would be difficult to carve out that type of more generous exception for the Defense Department, and likely even harder to get it approved for federal workers across the board.

Carter also wants to allow civilians to work part-time during the first year after a birth or adoption, and allow them to have more flexible work hours when possible. That plan may not require legislation.

The promotion proposal, which also requires a law change, would allow a major or captain to remain at their rank for years or even their entire career, if they are highly skilled in a critical field such as cyberwarfare or another technical job.

The new plans would also allow troops to ask to have their promotion review postponed if they haven't completed all the requirements for the next rank, and want to pursue another opportunity, such as an internship or higher education.

The current promotion system has been in existence for decades, and gives senior leaders little flexibility. Service members must complete a number of specific requirements — including certain command responsibilities and schooling — before getting promoted to the next rank. And they must do it within certain timelines.

Carter has complained that such systems tie leaders' hands and make it harder for them to compete for talent, particularly as he works to beef up innovation and technology within the department.

Another suggested change would allow the services to bring in new people with critical abilities and start them at a higher rank, rather than at the bottom of the officer pool. That is done now with some specialties, such as doctors, lawyers and chaplains. But Pentagon officials want to be able to do it for many high-tech jobs.

Other proposals would allow the military services to schedule promotions based on merit, rather than seniority, and make it easier for troops who leave the service for medical reasons to get civilian defense jobs.

And there are plans to upgrade recruiting efforts to make them more computerized and targeted, and also allow the department to hire more quickly when needed to get quality personnel.

The latest proposals come in the wake of changes Carter announced over the past seven months. In January, he doubled the length of fully paid maternity leave for female service members and expanded the hours that military child care facilities are open and the number of children that can be accommodated.

Last November, he rolled out plans to change the retirement system to allow investments in a 401(k)-type retirement plan and increase internships.

He has argued that the Pentagon needs to get in line more with the corporate world, and strengthen ties with high-tech companies to bring the best and brightest into the department.