The United States has been conducting unannounced Navy flight operations off Libya for the last several weeks, including fighter plane flights over the Gulf of Sidra but not beyond the "line of death" that Libyan leader Moammar Gadhafi has drawn across that waterway, Pentagon officials disclosed yesterday.

So far there has been no direct military response from Libya. But the official Libyan news agency JANA said late Friday that Libya's military will conduct missile firing exercises in the Gulf of Sidra, starting today. This would mark Gadhafi's first militant action since the U.S. bombing raid against his country on April 15 and could be the precursor of anti-U.S. action.

Since shortly after the April bombing, U.S. Navy carriers have been conducting the type of "freedom of navigation" exercises in the Gulf of Sidra that provoked Gadhafi's first military responses in March. On March 24, Libya fired six SA5 missiles at Navy planes but missed. Navy planes responded by firing antiradar missiles at the site and bombing Libyan missile boats.

The Reagan administration announced its intention to conduct Navy flight operations in the Gulf of Sidra before the April exercises commenced. In a change of policy, no similar advance notification was given to Libya or the international aviation community for Navy maneuvers and flight operations in the gulf since the April bombing.

"That earlier notice was a courtesy," said one official. "But when somebody shoots at you, you don't owe him that courtesy."

In recent weeks, fighter planes from the carrier USS Forrestal have flown to within about 40 miles of the "line of death" Gadhafi drew across the Gulf of Sidra -- far out to sea from the 12-miles-from-coast limit that the United States recognizes. The Forrestal has gone into port, but the carrier USS America is expected to continue the U.S. naval presence in the gulf.

Sources said the intermittent flights north of the "line of death" are designed to reassert the U.S. right of exercise in those international waters, to remind Gadhafi of U.S. retaliatory capabilities in hopes it will deter him from supporting new acts of terrorism, and to escort electronic eavesdropping planes that collect data for the U.S. intelligence community, particularly the National Security Agency.

Until the JANA report late Friday, Libya had not made a public issue of the resumption of U.S. naval operations. Administration officials said Gadhafi seemed to have been stunned into inactivity by the April bombing. They said the Soviet Union also has cautioned Gadhafi against adventurism and terrorism.

The Kremlin leadership, officials added, also has been restrained in sending new weaponry to Libya. Soviet technicians, they said, have strengthened Libya's air defenses, but the Soviets have not delivered sizable quantities of offensive weaponry.

The main Soviet effort, officials said, has been improving the antiaircraft defenses provided by the SA5 missile sites at the coastal cities of Surt and Benghazi. Not only has the Benghazi site become operational since the April raid, sources said, but it has been linked by computer to the Surt SA5 missile site in hopes of making the coastal air defense more effective. Libyan gunners shot down only one U.S. bomber in the April raid.

In addition, sources said, the Soviets within the last few weeks have sent SA2 antiaircraft missiles to sites in and around the capital city of Tripoli.

The JANA news agency, besides announcing the upcoming missile firing exercises, quoted "informed diplomatic sources" as asserting that the United States is moving Air Force AWACS (airborne warning and control system) from West Germany to the Mediterranean region, and that "a number" of B52 bombers had landed at the British base in Mildenhall.

Air Force officials refused to comment.

Informed sources said that Navy planes in the most recent Gulf of Sidra operations have been flying in a nonprovocative manner, such as following well-defined courses rather than feinting toward Libya. The idea, officials said, is to project the image of conducting "normal" flight operations over international waters, not to twit Gadhafi.