The Army has told Defense Secretary Caspar W. Weinberger in a secret report that almost 100,000 more soldiers will be needed to carry out President Reagan's military strategy and that it doubts they can be recruited under "the volunteer concept," suggesting a draft may be required.

The report is the Program Objective Memorandum detailing the Army's manpower and weapons plans for fiscal years 1983 through 1987 and requesting money to carry them out.

Citing Reagan's plan to expand the Rapid Deployment Force -- designed to rush to emergencies in the Persian Gulf -- as part of new demands for officers and troops, the report said the Army should grow to 870,000 by fiscal 1987. That would be about 96,000 above its current force and 83,700 more than the number expected in uniform by the end of fiscal 1982.

The Army's civilian work force also would expand under the new five-year plan, from 381,700 in fiscal 1982 to 412,900 in fical 1987.

In discussing its plan to field a force of 870,000 by 1987, the Army report said: "This growth is necessary to support substantial force structure increases and will require extraordinary manpower policies to include significant augmentation to the Volunteer Concept."

Although the words "draft" and "conscription" were not used in asking for such augmentation, several Pentagon sources said the Army is counting on a draft eventually, whether or not that is offically admitted.

Asked if what he has called the "new" military strategy will require conscription, Weinberger said through a spokesman yesterday that "the draft is not anything anybody is considering." Although he could disapprove any Army requests to resume the draft, Weinberger has left himself the loophole of resorting to it if the all-volunteer system fails to attract enough soldiers.

The volunteer force replaced the draft in 1973 as part of the backlash after the Vietnam war. The military services are currently recruiting enough volunteers to fill their ranks, although leaders have warned that the pool of available young men and women will shrink in this decade. If the economy improves, fewer young people are expected to join the military since civilian jobs will be available.

The volunteer force has been criticized heavily in Congress, with a growing number of lawmakers contending that it costs too much, puts too much of the defense burden on minorities and the poor and is providing 9 a.m.-to-5 p.m. soldiers who see the military as just another job, not an obligation.

Although Reagan opposed draft registration and the draft during his campaign, he has not moved to repeal the registration requirement enacted under President Carter. To avoid a return to the draft, Reagan has vowed to improve life in the volunteer force through higher pay and other benefits.

The administration yesterday announced the formation of a high-level manpower task force that officials acknowledged could lead to recommendation of some type of draft to augment or replace the volunteer force.

"We're not prejudging anything," an administration official said when asked if the task force, to be headed by Weinberger, may recommend a return to the draft.

The official said the other military services also will need more people to carry out the administration's military strategy. He estimated a 10 percent increase would be required by 1985, meaning 200,000 additional officer and troops for the Army, Navy, Air Force and Marine Corps unless civilians or reservists fill some of the active-duty billets. The United States now has 2,031,395 men and women on active duty.

Executive director of the manpower task force is Maj. Gen Thomas K. Turnage, designated director of the Selective Service Sytem, which would run any conscription program.

Other members are budget director David A. Stockman; presidential counselor Edwin Meese III; Richard V. Allen, national security adviser; Air Force Gen. David C. Jones, chairman of the Joint Chiefs of Staff; the secretaries of the military services; Martin Anderson, assistant to the president for policy development, and Murray L. Weidenbaum, chairman of the Council of Economic Advisers.