The American Civil Liberties Union sued on Tuesday on behalf of four U.S. servicewomen to challenge a longstanding policy barring women from thousands of ground combat positions, citing the changing nature of warfare and fairness for career soldiers.

The civil rights group argued in a legal complaint filed in federal court in Northern California that a military policy to bar women from combat roles on the basis of gender was unconstitutional.

"Nearly a century after women first earned the right of suffrage, the combat exclusion policy still denies women a core component of full citizenship - serving on equal footing in the military defense of our nation," reads the suit, on behalf of four women soldiers who have fought in Iraq or Afghanistan.

Their career opportunities also had been limited by the policy, the women said.

The lawsuit comes as the Department of Defense has slowly been dropping such gender-based restrictions. In February, it allowed some women to serve in combat battalions, a unit of 300 to 1,000 members, and dropped restrictions on women serving in units that were required to be based with combat units.

But women are still not allowed in infantry, or in smaller units engaged in combat. Women are barred from more the 238,000 positions, the ACLU said. But in Iraq and Afghanistan, where there are no clear battle lines, women have been pulled into combat in spite of the policy, the group added.

Asked about the lawsuit at a briefing, a Pentagon spokesman said that Defense Secretary Leon Panetta remained "very committed to examining the expansion of roles for women in the U.S. military and he's done so."

"On his watch, some 14,500, give or take a few, positions have been made available to women. And he has directed the services to explore the possibility of opening additional roles for women in the military," spokesman George Little said.

"So I think his record is very strong on this issue. The recent openings that I just referred to are merely the beginning and not the end of a process," he added.

OPENS UP OPPORTUNITIES

A 2009 study by the Defense Department found that women had already served in combat roles in the two wars, and that female veterans felt the experience opened up career opportunities.

Some defense officials have noted that 10 years of combat had made it clear that some gender-based restrictions were obsolete because battlefields faced by U.S. troops in Iraq and Afghanistanhad no clear front lines and no obvious ways to limit exposure to fighting.

Under current policy adopted in 1994, women are allowed to serve in combat units as medics, intelligence officers and other jobs at the brigade level. But women cannot be assigned to perform the same job in a battalion, which can be as small as a few hundred troops.

The military has sometimes gotten around the rules by temporarily attaching women to battalions, which allowed them to work in the smaller units but kept them from officially receiving credit for being in combat.

Since combat experience is a factor in promotions and job advancement in the military, women have had greater difficulty than men in moving up to the top ranks, officials said.

The Air Force is the service most open to women, with no gender restrictions on 99 percent of the jobs. The Marine Corps and U.S. Army are more difficult, barring women from more than 30 percent of jobs for enlisted personnel, mainly combat and armor positions, the Defense Department said.

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The ACLU and law firm Munger, Tolles & Olson filed the request for an injunction on the policy in the U.S. District Court, Northern District of California. The lawsuit names Panetta as a defendant.

(Additional reporting by David Alexander in Washington; Writing by Peter Henderson and Cynthia Johnston; Editing Paul Thomasch and Cynthia Osterman)