The U.S. Air Force has established a secret, contingency air base in an unpopulated part of Egypt and has 100 airmen stationed there, according to informed sources and congressional testimony.

Military supplies totaling about $70 million are stored at the facility, which a Pentagon official told Congress is "in the middle of nowhere" and thus "is a very good base for secret operations."

The base, upgraded last year, has been used for deployment of AWACs (Airborne Warning and Control System) planes and for training missions to the Middle East, and could "support in certain contingencies up to perhaps two tactical air command fighter squadrons," according to testimony this year.

It is one of several American-built bases that the Reagan administration has initiated overseas, reversing a trend toward reducing the number of U.S. installations abroad.

The administration is developing similar contingency installations in Morocco, Turkey, Liberia and Honduras. It also tried to persuade Colombia to accept one.

Differing from earlier bases, many have few, if any, U.S. servicemen and almost no dependents.

Nonetheless, this growth in the number of installations abroad has drawn criticism from the House Appropriations Committee, which this month said it was "concerned that this unilateral buildup is not the product of an overall strategy, but rather a series of isolated responses to separate emergent situations."

The Egyptian base drew special criticism because it was upgraded with $7 million in operational funds, not military construction money, a move that committee members said kept them uninformed of its existence.

The Air Force is considering expanding the base with a permanent dormitory and eating and medical facilities costing $10 million.

The existence of the base was kept secret at the request of the Egyptians, according to the House testimony. Committee members were told there was no "formal public agreement" and the Egyptians "will not give one" permitting continued American use of the facility.

The base "is a matter of great sensitivity with them because there is a continuing presence of U.S. troops there," Maj. Gen. Richard V. Secord, deputy assistant secretary of defense for Near Eastern and South Asian affairs, told the House Appropriations subcommittee on military construction in February.

The identification of Egypt as site of the base was deleted from the published transcript of the hearings, but was provided by sources.

The secret base has taken on added importance with the failure of U.S.-Egyptian negotiations on construction of a large, permanent base at Ras Banas for the U.S. Rapid Deployment Force.

In Central America, the administration wants to develop two airfields in Honduras for contingency use. Runways at both Comayagua and La Ceiba are to be widened and radar and fuel depots added to accommodate any U.S. Air Force plane. No materiel would be stored at the bases and no personnel would be permanently stationed there, according to testimony given Congress.

"Support facilities such as maintenance shops, mess halls and sleeping facilities would be installed when the need to use the airfield arose . . . ," according to Nestor D. Sanchez, deputy assistant secretary of defense for inter-American affairs.

Work on these bases would be in addition to construction, now under way, of a training facility in Honduras for Salvadoran troops and the completion of a large radar center, staffed with about 45 Americans.

Congress agreed to provide $14 million for Comayagua last year. The House Appropriations Committee this month refused the $8 million requested for La Ceiba, saying the Comayagua facility "will provide sufficient access into the region." The House voted Tuesday to delete the funds.

Administration officials did not fight the House decision, but are expected to push harder for La Ceiba in the Senate. Last year the House rejected funds for both facilities, but the administration won support for Comayagua in a House-Senate conference.

In Morocco, the administration is planning to spend $25 million next year on runway improvements so that a base previously used for B47s can accommodate heavy transports, such as the C5 and the C141, "en route to Egypt and coming back from the theater," according to testimony given the subcommittee.

In Liberia, the United States signed an agreement in February to upgrade its international airport at Monrovia.

The United States also has agreed to build facilities at Batman and Mus in eastern Turkey. One of the facilities is 150 miles from the Soviet border.