Assistant Secretary of State Thomas Enders said yesterday that the Reagan administration is not encouraging an increase in defense spending by Latin American countries and that recent administration efforts to relax restrictions in arms sales and to supply advanced warplanes to those nations result from a U.S. interest in not allowing other powers to become the region's chief suppliers.

Enders, testifying before a House Foreign Affairs subcommittee, said that a number of Latin countries are seeking to upgrade their warplane forces, but that the administration believes that the possible sale of F16 fighters to Venezuela would not result in a South American arms race. He added that no other Latin American nation had approached the United States about F16 purchases.

The administration tentatively has decided to sell Venezuela 24 of the advanced planes for about $500 million, breaking a longstanding U.S. policy of not introducing such sophisticated weapons into Latin America.

Enders, the department's chief of inter-American affairs, defended the proposed sale as justified in part by Cuba's stockpiling of advanced planes, and argued that in the past "U.S. efforts to prevent introduction of weapons deemed sophisticated have failed in every case. All we did really was weaken the security relations we had" in the region.

Enders said Venezuela faces a potential threat to its Caribbean oil-export routes from the Soviet Mig23 fighter-bombers maintained by Cuba. Potential bases for these planes are being constructed in Granada and Nicaragua, Enders said, adding that Nicaragua is believed to be training Mig pilots.

The United States was once the largest supplier of Latin American arms, but since 1974 has fallen into fifth place, behind West Germany, France, Israel and Italy. "It is not in the U.S. interest for military services of our closest neighbors and hemispheric allies to be predominantly equipped by other countries," Enders said.