The United States has sent four airborne warning and control system (AWACS) surveillance planes to Egypt and had dispatched temporarily the aircraft carrier Nimitz to Mediterranean waters near Libya in a move that administration officials said last night was a warning to Libya not to move militarily against neighboring Sudan.

At his news conference last night, President Reagan confirmed that the planes were in Egypt. But, apparently in line with an administration decision not to announce the purpose of the move, he said they were there for routine joint exercises with Egyptian forces.

The president also caused some confusion when, in response to questions about the Nimitz, he said, "I don't believe that there's been any naval movement of any kind."

However, after the news conference, Pentagon officials said the Nimitz had been moved earlier in the week from supporting U.S. Marines in Beirut and had been conducting training exercises in the central Mediterranean until last night, when it headed back to Lebanese waters.

They added that planes from the carrier had flown over the Gulf of Sidra, whose waters are claimed by Libya but are regarded as international by the United States.

In August, 1981, two U.S. Navy jet fighters shot down two Soviet-made Libyan jets when the American planes were conducting maneuvers over the gulf, which is formed in the southern Mediterranean. However, the Pentagon officials stressed last night that the recent Navy flights were too far away from the Libyan mainland to be considered threatening to the forces of Libyan leader Muammar Qaddafi.

Underlying the confusion, U.S. officials said, was concern that Qaddafi, whom the administration has branded "an international terrorist," has staged a buildup of aircraft in the southeast corner of his country and may be planning an attack against Sudan, an ally of Egypt and the United States.

To warn Qaddafi off, the officials continued, Reagan responded to a request from Egyptian President Hosni Mubarak for a show both of naval strength in the area and use of the AWACS planes to monitor the area where the borders of Libya, Sudan and Egypt meet.

These planes are equipped with sophisticated electronic equipment able to scan the air for hundreds of miles and can function as "flying command posts" capable of directing air battles.

However, the officials stressed, while the AWACS will be used to detect any Libyan plane movements threatening Sudan, any response against the Libyans, in the event of actual combat, will be the job of the Egyptian air force. Egyptian air force personnel also are included in the AWACS crews, the official said.

Publicly, the administration sought to take a position intended to give a strong hint of its intentions without specifically accusing or threatening Qaddafi. That was implicit in a statement issued by the State Department last night, which said:

"We have long been aware of Libyan efforts to destabilize governments in the past. These efforts are well known to us and our friends. The current deployment of U.S. Air Force AWACS aircraft is in concert with combined U.S. and Egyptian air training. These exercises have been conducted in the past and will be repeated in the future.

"Our fleet units in the Mediterranean are deployed at normal peacetime readiness levels and, as always, are prepared to support contingencies as necessary."

Reagan, at his news conference, took a similar line. He insisted the presence of the AWACS planes in Egypt was "not an unusual happening." But he also noted pointedly, "We're well aware of Libya's attempts to destabilize its neighbors and other countries in that part of the world."

There is a long history of animosity between Qaddafi and Sudanese President Jaffar Nimeri that has included Libyan bombings of Sudanese border areas where rebels from another neighboring country, Chad, had been operating against a Qaddafi-supported faction in their country.

In October, 1981, following the murder of Egyptian President Anwar Sadat, the United States sent AWACS planes to Egypt, and their presence then was credited with helping to halt the Libyan attacks against Sudan.

More recently, Nimeri has been under heavy domestic pressure resulting from a faltering economy and the threat of a civil war in the south of his country. As a result of the unsettled conditions in Sudan, U.S. officials said, Qaddafi may have decided that the time was ripe for another attempt at overthrowing Nimeri.