President Reagan yesterday notified Congress that two U.S. airborne warning and control system (AWACS) surveillance aircraft and eight F15 fighter planes had been deployed in Sudan for a "limited" but undefined period to help Chad "and other friendly governments assisting it" in their escalating conflict with Libyan-backed rebels.

Reagan's message, in a letter to House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) and Sen. Strom Thurmond (R-S.C.), president pro tempore of the Senate, was made public as Libyan aircraft reportedly resumed bombing a key Chadian government stronghold after a three-day lull.

Libya, meanwhile, according to its government news agency, Jana, ordered its air force to attack the AWACS planes if they "in any way, have effect over Arab Libyan territories."

Administration officials said yesterday that the question of Libyan air raids would be a major factor in French and American decisions on how to respond further to the crisis in the central African country.

The Pentagon said that 550 U.S. personnel, including ground support forces, were in Sudan in connection with the Chad mission.

Both the White House and the State Department characterized the situation in Chad as "serious" and State Department spokesman John Hughes said, "If reports of new bombing . . . are accurate, you would have a very serious situation."

Hughes indicated that the two AWACS planes, still on the ground in the Sudanese capital of Khartoum, might be used in a supporting role if the French government decides to deploy combat aircraft to deter future Libyan bombing raids inside Chad.

"It is obvious that what decision the French take would be a factor" in deciding whether to use the AWACS, the spokesman said.

U.S. officials yesterday spoke with satisfaction of France's readiness to consider greater involvement in Chad, and said that its reported refusal on Sunday to intervene directly in the crisis had been "considerably modified" in diplomatic contacts with Washington.

Hughes said that between 1,500 and 2,000 Libyan troops, equipped with artillery and armored vehicles, have besieged the northern Chadian oasis town of Faya Largeau. He suggested that Libyan air attacks may have been halted over the weekend because of concern in Tripoli about French and American responses.

He said that the Libyans could be "in no doubt" about U.S. concern over Libyan intervention in Chad, which the administration has called "open aggression" even though there has been no contact between the two countries.

He also said that the three U.S. advisers sent to Chad to train troops in the use of Redeye antiaircraft missiles had completed their assignment and had left or were about the leave the country.

White House spokesman Larry Speakes, meanwhile, disclosed that the United States has "been in contact with the Soviets with regard to our views of Libyan intervention in Chad," but he said there was no evidence of Soviet involvement.

In Moscow, the Soviet news agency Tass accused the United States of capitalizing on the war. "American interference increasingly threatens to develop into an act of intervention directed both against the forces of the transitional government of national unity of Chad and against neighboring Libya," Tass said.

Notifying Congress of the aircraft deployment under the War Powers Act, Reagan said the U.S. forces "will be available to operate in close coordination with Chad" and its allies. "The mission of the F15 aircraft, which are equipped for combat, is to be prepared to provide protection to the other United States aircraft, if necessary," he wrote.

The War Powers Act requires the president to notify Congress within 48 hours of sending combat-equipped forces into a foreign country or substantially enlarging the number of U.S. troops in place.