The Reagan administration has become increasingly fearful of the possibility of military defeat at sea and is ordering the services to take new steps to prevent this, according to secret policy guidance now circulating in the Pentagon.

Defense Secretary Caspar W. Weinberger in the final draft of "this first complete Defense Guidance of this administration" to the military services sets forth these new policy contours and marching orders for the five-year period 1984 through 1988:

"It will be U.S. policy that a nuclear war beginning with Soviet nuclear attacks at sea will not necessarily remain limited to the sea," states the guidance.

Critics long have contended that it makes no sense to keep building nuclear aircraft carriers, costing $3.4 billion just for the ship itself, when both the United States and Soviet Union have perfected nuclear-tipped antiship missiles and torpedoes that could overwhelm any defense and sink the carrier. The Reagan administration, which has decided to keep building such carriers, is trying to build a firebreak between the use of conventional and nuclear weapons at sea.

By holding out the threat that sinking an American carrier with a nuclear weapon might cost the Soviets Moscow, the administration is building an addition to the long standing policy of "mutual deterrence." Reasons the guidance: "For the Soviet Union the prospect of losing their fleet to U.S. naval theater nuclear forces may not be sufficient to deter the Soviets from initiating a nuclear campaign at sea."

The Air Force and Navy must to some extent meld their traditional roles and missions and join together both to keep tabs on the Soviet fleet and other potentially hostile navy forces and, in a war, sink them.

A combination of Air Force bombers, starting with existing B52s, and cruise missiles, some of them based on Navy ships, must be exploited to achieve "clear maritime superiority," according to the policy guidelines. Years ago Congress approved a plan to arm Air Force B52 bombers with Harpoon antiship missiles, but the program became a casualty of budget cutting and Air Force-Navy rivalry.

Against the backdrop of the sea battles raging in the Falklands, the Reagan administration's guidance now circulating in the Pentagon stresses land- and carrier-based planes of the Air Force and Navy must be teamed up from now on for defense and offense, both at sea and over land.

"Navy force planning must include more utilization, when appropriate, of other service capabilities," states the guidance. "Specifically, the capabilities of Air Force AWACS airborne warning and control system and over-the-horizon radars in the North Atlantic to support" aircraft carriers and escorting warships "must be examined as complements to sea-based systems.

"The Air Force should plan on supporting" the Navy in keeping sea lanes open by flying planes equipped with antiship missiles from "key island and flank bases as part of an integrated air and sea superiority mission," says the guidance, in what some Air Force and Navy leaders regard as a shotgun marriage, with Weinberger holding the gun.

As part of the same demand to break down the traditional operational barriers between the Air Force and Navy, the new guidance states that "the ability of the Navy to contribute to land campaigns with long-range cruise missile and sea-based tactical aircraft and amphibious forces needs to be factored into the combined arms planning. This should include an analysis of naval forces availability and vulnerability in the different regions."

The guidance comes as good news to the Air Force's Strategic Air Command, which wants to employ B52G bombers armed with antiship versions of the Navy's Tomahawk cruise missile to patrol the oceans of the world. These bombers would become available for sea patrol as the new B1 enters the force, starting in 1986. B52 bombers, armed with a dozen of so medium-range antiship missiles, called MRASMs, fired from more than 100 miles away, would penetrate any known ship defenses, according to some defense planners.

The Air Force should consider designing a "worldwide deployable" team of fighter planes and communications gear to clear the skies of enemy aircraft threatening vital sea lanes between the United States, its allies and such supplies as oil.

The Mediterranean, Persian Gulf and Western Pacific are listed as "key geographical areas" for such an outfit. The way the Joint Chiefs of Staff intend to employ land-based aircraft from Iceland to cover the North Atlantic is cited as a model.

"Defense of vital lines of communication is an issue of particular concern because of the growing Soviet threat and the increasingly thin margin of forces available to cover commitments," states the guidance in making the case for a larger Air Force role here. "Therefore, the Air Force should consider using a limited number of long-range strategic bombers, with suitable radar and standoff weaponry, to support attacks on ships and Soviet facilities ashore that support naval missions. Planning for this role should include both nuclear and conventional weapons."

Although Pentagon leaders have not said much about it publicly, one justification they have given Congress for building the B1 bomber, which former President Carter canceled as obsolete against advanced anti-aircraft missiles, is its ability to sneak up on ships and hit them with antiship Harpoon and cruise missiles.

The B1 is much harder to see on radar than the B52. The radar-evading Stealth bomber under development is also seen as an even more lethal ship-killer in this era when "smart" weapons are posing an unprecedented threat to surface ships.

"The environment of future warfare is likely to differ greatly from any we have known in the past," notes the Pentagon guidance to the services. "Our principal adversary will have acquired, a decade hence, the capabilities to keep our land-based surface and naval forces under near constant surveillance . . . . Combat against Soviet forces, and possibly Soviet-supplied forces, will be of higher intensity and longer duration with weapons of much greater accuracy and possibly higher rates of fire and mobility."