Combat jobs across the US military are set to open up to women by next year, but officials don't expect huge numbers to end up in the posts.

In January 2013, at the request of US President Barack Obama, the Pentagon made the historic move of ordering all branches of the armed forces to open up ground combat jobs to women by 2016.

General David Perkins, who heads up the US Army Training and Doctrine Command, said military studies and a look at the experience of Canada, which has for years allowed women in combat, showed few women are likely to choose such roles.

"One of the biggest challenges, from an implementing point of view, would probably be a turning of small numbers," Perkins told reporters on Tuesday. "It just probably won't be a lot."

The Pentagon decision to allow women in combat overturned a 1994 ruling barring women from units with a direct ground combat role.

The exclusion has often been cited as a bar to female officers reaching the highest military ranks where combat experience is considered an indispensable qualification.

But advocates of opening combat positions to women have argued that they already are exposed to the perils of combat in modern conflicts with no clearly defined front lines.

Since the decision to open up combat specialties to women, 91,000 positions out of a total of 331,000 posts previously closed to women have been opened, according to Defense Department data.

The Army, Marine Corps, Air Force and Navy can still ask for women to be excluded from certain jobs, but such exemptions will have to be justified.

Currently, 15 percent of all US military personnel -- combatant and non-combatant -- are women.

Women can now operate the Multiple Launch Rocket System or Javelin anti-tank missiles, maintain the Abrams Tank System, or be mine detection dog handlers -- all roles they were previously barred from.

But the numbers of women performing those tasks may not grow dramatically, Perkins said.

- Physical challenges -

One reason is that women are "dramatically less inclined" to choose infantry, armored units or artillery than their male counterparts, he said.

Another challenge is the gender neutral physical fitness tests women must pass in order to qualify for combat.

No females have successfully completed the Marines' infantry officer training, for example, one of the toughest physical tests in the military today.

Similarly, no women have passed the army's elite Ranger School course, though three have been given the green light to try again on the previously men-only course.

For most women, the main concern is achieving the physical prowess of their male counterparts -- and not seeing the standards change, according to Gayle Lemmon, author of "Ashley's War: The Untold Story of a Team of Women Soldiers on the Special Ops Battlefield."

"When you talk to female soldiers, the things they would say first and foremost is that they do not want to see a (physical) standard change," she told AFP.

"They don't think that anything could be more harmful, because it could be seen as a lowering of the standards."

She said she is hopeful that some women will one day be shoulder-to-shoulder with their male colleagues when it comes to passing those tests.

"I do think women will meet that standard," she said.

- Similar to Canada -

The United States is not alone when it comes to including women in military combat roles.

In Canada, for example, where women were permitted to join combat ranks as early as 1989, their numbers in those roles remain low.

Women make up 0.5 percent of infantry, two percent of armored units and four percent of artillery, according to Perkins.

Perkins said it is important to integrate women into units together, so they do not feel isolated.

"We would want to have not just (female) privates in a battalion, probably you would want also some (female) non-commissioned officers and officers," he said.

But he estimates females will remain underrepresented.

"We won't have enough (women) for one per battalion, we know that," Perkins said.