Defense Secretary Robert Gates says the day will come when women will be allowed to serve in special operations units, the kind of commando teams that conduct secret raids and nab terrorists.

Such an allowance would be a major change for the military. Women by law aren't allowed to serve on the front lines of combat, although women serving in Iraq and Afghanistan often face the same threats as infantry troops.

Gates was addressing a group of ROTC students at Duke University on Wednesday when he was asked whether special operations units will open to women now that they are allowed on submarines.

Gates said he expects women will eventually be allowed to join special operations forces in a careful manner, as they are on subs. Only female officers are being allowed on submarines, and only on subs large enough to accommodate separate living quarters.