The Defense Department has turned down efforts to pull married couples with children and single parents out of the war zone, saying it would undermine the troops.

Defense Secretary Dick Cheney and Gen. Colin Powell, chairman of the Joint Chiefs of Staff, said Thursday it would be a serious mistake to change the complexion of the fighting force.In a joint letter, Cheney and Powell said withdrawing these two groups "would weaken our combat capability by removing key personnel from our deployed units and by undermining unit cohesion and esprit de corps.

"It would also break faith with our single parents and military couples and their comrades who depend on them every day," they wrote.

Although the Pentagon does not yet have the number of married couples and single parents in Operation Desert Storm, there are in the 2.1 million strong military 54,021 couples, 37,267 single men parents and 17,836 single women parents.

The Pentagon did not have a breakdown on how many of the couples have children.

The letter was requested by Senate leaders after the issue was raised during a private briefing that left at least one senator, Sen. John Heinz, R-Pa., considered it important for the Pentagon to review regulations in this area.

Heinz insisted Powell said the review was needed "because he did not want - and I quote him more or less directly - `a married couple with children in the same foxhole' because he is very sensitive to the idea of both parents being killed."

But in their letter, Cheney and Powell were explicit in their opposition to any change in the current policy that has sent married couples and single parents into the war theater.

"We understand and appreciate your concern," the letter said. " . . . We urge you, however, not to allow that concern to lead you and your fellow senators to call for a policy that, in our view, would be both unwarranted and unwise."

Cheney and Powell said that all members of the armed forces are volunteers. "They understand when they volunteered to serve, they freely assumed the duty and obligation to place themselves in harm's way when called upon to do so."

They said married couples and single parents served in Korea and Europe as well as in the invasions of Grenada and Panama.