The United States is planning to withdraw its Airborne Warning and Control System (AWACS) radar planes from the conflict in Chad because they have "served their purpose," an administration official said today.

The official said that the announcement could come as soon as Tuesday.

However, White House spokesman Larry Speakes, questioned today about the possible withdrawal of AWACS planes, said that no final decision has been made.

Before any such decision, he said, U.S. allies and Congress would be consulted.

The Reagan administration sent two of the sophisticated AWACS surveillance planes to Sudan, Chad's eastern neighbor, earlier this month to assist Chadian forces in fighting an invasion by Libyan air and ground forces.

The United States has also sent eight F15 fighters to the region. It could not be learned today whether they will also be withdrawn.

The official said the AWACS planes have "achieved their purpose" by getting French forces involved in the fighting, by contributing to the current halt in the Libyan advance and by preserving the government of Chadian President Hissene Habre.

The withdrawal would come after the French sent nine combat aircraft to Chad to support French troops aiding the Habre government against rebel forces backed by Libya. On Sunday, the French sent four Jaguar combat jets, an air tanker and four F1 Mirages to the Chadian capital of Ndjamena.

In Washington, a Pentagon official said that the Defense Department has been working for several days on plans to recall the AWACS planes but that the final order has not been given. He said that he believes the order is imminent and that the planes probably will be withdrawn at the end of the week.

The official said the decision was not made in anger at the French.

" . . . If they have no projected need for them, we have no reason to keep them there," he said. "It's expensive to keep them over there, and it ties up an awful lot of people."

Another administration official said plans to withdraw the AWACS planes were set about a day ago but might be changed.

The unarmed AWACS planes are used to spot and track aircraft as far away as 200 miles. And AWACS radar operators can guide fighters to enemy planes in an aerial intercept system. The F15s would be their fighter screen against attack.

Libyan leader Col. Muammar Qaddafi had threatened to shoot down any AWACS planes supporting Chad.

Until the arrival of the French aircraft, the Chadian army had been without air support to counter the reported extensive bombing and strafing by Libyan planes supporting the rebels.

Habre, whose nation has no air force, had asked repeatedly for French aircraft, and U.S. officials have pressed the French to provide them.

Reflecting apparent U.S. dismay at what it perceived as a slow French response to the Chadian conflict, President Reagan said Aug. 11 that he believed Chad was within the French "sphere of influence" and that he hoped the government of President Francois Mitterrand would respond accordingly.

He also ruled out any direct U.S. military intervention.

Reagan's statement regarding France touched off a somewhat acrimonious, but indirect, public exchange between Paris and Washington.

Last week, Mitterrand let it be known through an interview with the French newspaper Le Monde that he was "irritated" with the Reagan administration and believed that U.S. involvement had complicated the situation in Chad and made resolution of the conflict more difficult.

Following that report, the administration responded that France had been consulted "at the highest levels." And U.S. officials disputed Mitterrand's assertion that the AWACS had been sent to Sudan before the French had been informed.

This week Mitterrand is expected to give what French officials have again described as an "interview" discussing France's intervention in Chad following shipment there of the combat aircraft.

Although the forum and timing of the interview have not been announced, French officials in Paris said today that it is likely to be released Wednesday. It is expected to be a more formal statement of French policy in Chad and on domestic economic and political matters.