The Carter administration has ordered two U.S.-piloted radar reconnaissance planes to Saudi Arabia in its most direct demonstration so far of determination to counter military threats to the area's stability, Defense Department officials said yesterday.

The Airborne Warning and Control System (AWACS) aircraft are capable of monitoring flights of enemy planes, such as the Soviet Mig21s that North Yemen claims have flown raids against it from South Yemen.

A Defense Department official described the AWACS as a "passive system, not an attack plane." and said the planes will remain "over Saudi Arabia." But he added that the deployment was part of an "overall response to [threats] to the area's security." The planes, from Okinawa, are scheduled to arrive in Riyadh today.

In the meantime, the Pentagon began spelling out details of a broad program to strengthen the U.S. air and sea presence in the Mideast area at a time when Saudi Arabia, the world's largest oil exporter, feels increasingly threatened by events unfolding around it.

Defense Department spokesman Thomas Ross said that the United States has offered to send a squadron of F15s to Saudi Arabia on a temporary basis and is considering establishing a permanent fleet in the Indian Ocean. The aircraft carrier Constellation, accompanied by a guided missile destroyer and a guided missile cruiser, left the Philippines Wednesday night en route to "operations" in the Arabian Sea, the Pentagon said.

Aboard the Constellation are 5,000 service personnel and 80 aircraft, including F14 fighters, which have a range of 750 miles.

Congressional reaction to the U.S. moves and proposed moves in the region has been muted.

Several congressional sources yesterday privately questioned the wisdom of dispatching advanced equipment and Americans to Saudi Arabia only weeks after the collapse of the Iranian regime, which had received large amounts of U.S. military hardware and technical support.

Under a $2.5 billion arms sale approved by Congress in May 1978, Saudi Arabia is to start receiving 60 American F15s fighters in 1981. At present, 57 F14 aircraft bought by the shah of Iran before his downfall are under the control of the Iranian junior officers who dominate that country's armed forces. Pentagon officials express concern that the security of these aircraft and classified documents has been compromised.

Sen. Frank Church (D-Idaho), chairman of the Senate Foreign Relations Committee, has been sharply critical of Saudi Arabia's unwillingness to support the Camp David Middle East accords. He has suggested that the timing may be right to press Saudi Arabia for concessions.

The Yemeni crisis erupted into border warfare when South Yemen tanks and troops drove deep into North Yemen's border provinces and have remained there despite Arab League calls for with-drawal.

North Yemen radio broadcasts charged Wednesday that Mig21 fighters from South Yemen twice raided defensive positions in the Qa'tabah area.

Defense Department officials declined yesterday to provide details on the extent of Soviet or other communist advisory or military presence in South Yemen, which borders on Saudi Arabia.

The Saudi Arabian government approached the administration about working out a plan that would permit the Saudis to send a squadron of F5E interceptors and heavy artillery to North Yemen to ceptors and heavy artillery to North Yemen to counter the attack. Jordan and Egypt reportedly have offered to send advisers to help North Yemen.

U.S. law provides that arms sold to foreign countries may be used only for self-defense or for internal security. However, it provides for their use under some circumstances falling within the United Nations charter when a neighboring country requests help in staving off an attack.

Pentagon officials said that events in the Yemen war over the weekend could determine whether to dispatch the armed squadron of U.S. F15s to Saudi Arabia. It was understood that one reason for the delay is to give the Saudi government time to inform neighboring countries.

Ross stressed the deployment would be temporary. Defense Department sources said the planes would offer a show of wupport while freeing Saudi aircraft for other missions.

An unarmed squadron of U.S. F15s was in Saudi Ariabia from Jan. 13 to 22.

Ross also cautioned yesterday that the option of establishing a permanent fleet in the Indian Ocean was still in the stage of being "brainstormed" by Pentagon experts.

He said that the carrier task force steaming toward the Indian Ocean from the Philippines was not intended to be the beginning of such a fleet.

The United States currently operates a "naval support facility," leased from the British, on the remote Indian Ocean island of Diego Garcia. The airfield recently has been lengthened and the facility can provide an anchorage for a small naval task group, but not a fleet.

Congressional sources noted yesterday that one factor in whether to form an Indian Ocean fleet is talks begun last year with the Soviets on the Indian Ocean.