WASHINGTON - Troops in Iraq and Afghanistan currently fighting beyond their enlistment contract could soon be getting a monthly bonus, but exactly how much and when has not been determined.

Rep. John P. Murtha (D., Pa.) said yesterday that Army officials were working out details about how to distribute the payments Congress mandated to about 12,000 troops now experiencing "stop-loss" extensions of their service. He is pushing for the payments to be $500 a month.

Critics have called the military's ability to stop-loss troops, or keep them beyond their enlistment contract or retirement date, a back-door draft. Military leaders have said it was a necessary tool in wartime to keep troops with critical skills and maintain the continuity of a unit.

Lt. Col. George Wright, an Army spokesman, said the troops would receive the payments soon.

A law passed earlier this year said troops from all branches kept in the military under stop-loss in the budget year that started Oct. 1 would receive a bonus not to exceed $500 a month. It allocated $72 million for the bonuses.

Some members of Congress are also seeking a retroactive bonus for the troops who have been required to stay in the military beyond the time they enlisted since the Sept. 11 attacks. Murtha said there were about 185,000 total.