The nation's top-ranking soldier warned Congress yesterday that it may have to reactivate the draft to fill up the reserve forces.

Otherwise, Gen. Bernard W. Rogers told the Senate Armed Services Committee, the United States might have to go to war with no available backup for its standing Army.

Rogers, who became Army chief of staff in October, said in an interview after his committee appearance that before resorting to the draft he would like to try to entice people into the reserves by spending $750 million a year on inducements.

"But three-quarters of a billion is a lot of money," he said. "Congress may not want to spend that much money. Then we would have no other option" than to resume the draft.

The Army's basic problem - and that of other services - in regard to reserve forces is that men who joined reserve units rather than be drafted during the Vietnam war are finishing their tours and leaving in droves rather than re-enlisting.

On top of that, men who automatically became part of the standby reserve after finishing active-duty tours of two years or more during the Vietnam war are completing their six-year military obligation.

This exodus comes after the United States has adopted the "total force" strategy of assuming the reserves would be ready to go to war in a hurry to back up front-line troops from the active-duty Army. Reserve units have received billions of dollars' worth of modern equipment under this assumption.

Rogers said yesterday that the Army is not only short of reservists but could not count on getting trained draftees into the line until after a war had been in progress for seven months.

"This is a situation of serious proportions," Rogers said yesterday. U.S. military leaders have been warning for months that the Soviets might launch an attack against NATO forces with little warning and hit with blitzkrieg speed and punch.

While NATO forces on the line might be able to stop that first assault, Rogers said he was worried about the alliance's staying power, declaring that six months after such an attack his Army would be "320,000 short of what we need."

Stressing that he was not ready to give up on the volunteer Army, which he said has attracted "the finest soldiers I've served with in 34 years," the Army's new leader said the service must go all out in fiscal 1978 and 1979 to fill the gaps in the reserves.

Incentives the Army is studying for attracting people into the reserves include enlisment bonouses, income tax deductions of up to $1,500, money for education and retirement benefits. Rogers said the Army is still putting the package together.

If the incentive package is not approved or does not work if implemented, Rogers said that he sees no alternative to "some type of involuntary service."

The general said in an interview that he was not looking to the draft to increase the standing Army of 775,000 people, but only to fill vacanies in the reserves.

Under the plans being discussed, Rogers said, an individual might be drafted for an active-duty tour of six months or less or be required to train two weeks during the summer as a member of the standby reserve.

Pentagon figures show that the number of people in the National Guard and Army Reserve training units dropped from 621,000 at the end of the Vietnam war to 557,000 as of last June. Rogers said the standby reserve - called the Individual Ready Reserve and comprised of people who do not train every weekend - dropped from 977,000 in 1972 to 156,000 currently.