The Reagan administration is improperly using training exercises to build millions of dollars worth of facilities in Honduras that have not received the required congressional approval as military construction projects, the General Accounting Office has concluded.

The GAO, the investigative arm of Congress, said that many of the airstrips, barracks and other facilities constructed during three years of continual maneuvers appear permanent and operationally useful. The Defense Department has said they are temporary and useful only to train the soldiers who build them.

In a 35-page report that has not yet been published, the GAO said the administration reported $3.7 million in exercise-related construction in the Central American nation since 1983, which the GAO said significantly understates the real total.

In some cases, the military improperly manipulated figures to evade congressional reporting requirements, the GAO said.

"Clearly, the conclusion is that the Defense Department is continuing to fund its Honduran operations in a manner outside that which is prescribed by law," said a spokesman for Rep. William V. (Bill) Alexander Jr. (D-Ark.), who requested the GAO report. "The way they've manipulated the figures to make it appear legitimate is amazing."

A Defense Department spokesman said he could not comment because he has not seen the report. In the past, the administration has said that facilities are constructed in Honduras only to train Army engineers and thus are legitimate byproducts of military exercises, but the GAO disputed that rationale.

"This exercise-related construction has been used to support a continuous U.S. military presence in Honduras, carrying out a variety of training and operational functions," it said, adding that "even at the time of construction, a more extensive use . . . was contemplated."

Alexander and some other Democrats in Congress have expressed fears that the administration is using maneuvers to establish a permanent military presence in Honduras without congressional approval. Honduras is located between El Salvador, where the U.S. military is helping the Army fight a leftist insurgency, and Nicaragua, where the Central Intelligence Agency is supporting rightist insurgents seeking to topple a leftist government.

The administration, while saying it does not intend to establish permanent bases in Honduras, recently told Congress that it plans to build $50 million worth of facilities there during the next five years. But not all the facilities to be built during the course of exercises were specified in that report to Congress.

The GAO accused the military of evading reporting requirements, which apply to facilities costing more than $200,000, by breaking projects into components and treating each as a separate unit.

In a recent case, engineers built a stretch of road in northern Honduras but listed it as two connecting stretches, each costing less than $200,000, the report said.

"Both of these 'projects' were considered separate from a base camp constructed to support the entire effort," the report said.

In addition, the report said, the military had Honduras supply some of the fuel and other materials for the project so that their value would not be included in the total, then reimbursed Honduras for the materials.