Marine Corps leaders say they need two more years to study whether it makes sense to allow women on the front line.

They note that no woman has passed the even more challenging infantry training course for officers (10 have tried).

Before making a final decision, they said, they want to see many more woman marines try to pass the courses and evaluate the results.

"Any force-wide changes to be made will occur only after we have conducted our research, determined the way ahead and set the conditions to implement our recommendations," Captain Maureen Krebs, a Marine Corps spokeswoman, said.

In January, after years of debate and legal challenges, the Pentagon announced that it would lift its long-standing ban on women serving in ground combat units by 2016, unless the armed services can justify why certain positions should remain closed.

The decision was prompted in part by the recognition that women played a critical role in the wars in Afghanistan and Iraq, where commanders stretched rules to allow them to bear arms and support combat forces.

It remains an open question whether women will be allowed to try to become Army Rangers or Navy SEALs.

Army leaders say they are developing "gender-neutral" standards for infantry and other combat forces and plan eventually to open those units to women.

For the Marine Corps, however, the biggest obstacle to integrating women into the infantry may be overcoming a deep-seated cultural resistance to the idea.

Only 7 per cent of the Marine Corps is female, half the overall rate for the US military.

In a survey last year, one in six male marines said they would probably leave the service if they were forced to serve alongside women in ground combat units.

And although commanders say they are committed to giving women a fair shot, the corps isn't going out of its way to celebrate the fact that three woman marines passed the grinding training course for enlisted infantry.