WASHINGTON: Top US military leaders were expected to deliver their recommendations on Wednesday on opening combat jobs to women, teeing up a historic but delicate decision by Defense Secretary Ash Carter on how far to move gender boundaries in the world's most formidable military.

Carter has until year's end to decide whether to agree to a Marine Corps request that it be partially exempt from a January 2013 Department of Defense directive that military services allow women to compete for virtually all jobs, including the toughest and most dangerous roles on the frontlines of wars.

The services' recommendations, due by October 1, were not made public, but a US official confirmed that the Marine Corps had requested to keep some combat jobs open only to men. Officials from the other services - the Army, Navy, and Air Force - have hinted that they will not seek exemptions.

The decision could force Carter to choose between the views of his own top military adviser and advocates who say there should be clear, gender-neutral standards for combat roles that anyone can try to meet.

Gen. Joseph Dunford, the new chairman of the Joint Chiefs of Staff, recommended the Marine Corps exemption earlier in September, when he was still the service's commandant. In his new post, Dunford will have to approve any exceptions barring women from combat roles.

The stakes are high: some argue that including women in combat roles such as the infantry could upend, at least temporarily, the Marines' strong fighting culture.

"You have a cultural formula there that works. You've got Coca-Cola classic," said Elliot Ackerman, a writer and Marine Corps veteran who said he does not consider himself an advocate for either side. "If you add ingredients into that...it will not be the same formula...it will unequivocally change the culture."

If he agrees, Carter risks alienating those who say the time for equal treatment is long overdue. Opening ground combat jobs to women would strengthen the military, they argue, by ensuring the broadest possible pool of candidates.

"Certainly it's one of the last ways that women are treated differently in our law," said Elizabeth Gill, an attorney at the American Civil Liberties Union. "To have the door closed totally based on gender is unique in the federal government."

Carter - who has hinted he favours minimal, if any, exceptions - said Wednesday that he would "carefully review" reports from all four services and the Special Operations Command.

"Everyone who is able and willing to serve and can meet the standards we require should have the full opportunity to do so," he told a news briefing. "I am going to be very facts-based and analysis-based. I want to see the grounds upon which any actions that we take at the first of the year are going to be made."

Since 2013, about 111,000 military jobs have opened to women, mostly in the Army. About 220,000 jobs still remain closed, said Matthew Allen, a Department of Defense spokesman.

The greatest resistance has come from the Marines, an elite force that prides itself on its ability to react quickly and conduct frontline assaults.

It is the only service branch that has gender-segregated basic training. Just seven percent of Marines are women, compared with about 15 percent across the military.

As of March 2015, 25 percent of Marine Corps positions remain closed to women, compared with 18 percent of Army positions, two percent of Navy positions, and one percent of Air Force positions, according to a July GAO report.

Differences between military and civilian leaders on this issue have played out unusually publicly. After the Marine Corps released a summary of a study this month that found mixed-gender units performed worse in replications of ground combat than all-male ones, Navy Secretary Ray Mabus, who is in charge of the Marines, said he thought the study was flawed.

"That sort of public disagreement between a service chief and a service secretary, they happen but they're rare," said Captain Lory Manning, a Navy veteran and fellow at the Women's Research and Education Initiative, who favours allowing women in combat roles.

Even as more positions open to women, the gender make-up of the military leadership – drawn largely from those who have served in combat – will barely budge in the coming decades.

That is because it can take 25 to 30 years to reach the highest ranks, said Vice Chief of Naval Operations Admiral Michelle Howard, speaking at a think tank event in Washington last week.