The top U.S. Marine Corps and Army generals said on Tuesday that women should be required to register for the military draft, along with men, as the armed forces move toward integrating them fully into combat positions.

"I think that all eligible and qualified men and women should register for the draft," General Robert Neller, the commandant of the Marine Corps, told a Senate Armed Services Committee hearing on women in combat.

General Mark Milley, the chief of staff of the Army, also backed registration for women, although two other witnesses, Navy Secretary Ray Mabus and Acting Army Secretary Patrick Murphy, said only that there should be a discussion of legislation requiring women to register for the Selective Service.

The U.S. military is currently an all-volunteer force, but young men are required to register in case the draft is reactivated.

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The military leaders said it would take years for women to be fully integrated into combat units, although they generally voiced strong support for the plan to skeptical committee members.

Milley estimated full integration would take "no less than one to three years of deliberate effort."

President Barack Obama's defense secretary, Ash Carter, announced in December that the military would let women serve in all combat roles. The historic announcement was greeted with intense skepticism by many Republican members of Congress, as well as expectations it would require women to register for the draft.

Republican Senator John McCain, the committee's chairman, said at the time it would have "a consequential impact" on U.S. forces and their war-fighting capabilities.

On Tuesday, McCain said he worried there had not been enough planning. "I am concerned that the department has gone about things backwards," he said.

Many Republicans said they fear the imposition of quotas mandating a specific number of women in some units, such as Marines in positions that might require hand-to-hand combat.

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The military leaders repeatedly rejected that suggestion. "It would endanger not only the safety of Marines, but also the safety of our nation," Mabus said.

Obama's fellow Democrats expressed strong support.

Senator Jack Reed, the top committee Democrat, said physical abilities alone do not determine whether a military unit is effective.

"Fighting and winning wars, as I’m sure our panelists know well, involves much more than that," Reed said.