The elaborate U.S. air and sea surveillance net in Central America has detected a slowdown in arms shipments from Nicaragua to El Salvador, which could be a diplomatic signal to President Reagan but more likely is just a pause in the flow of arms, administration officials said yesterday.

One top official who has been studying the latest intelligence reports, gathered by Air Force planes and two Navy ships, did not rule out the possibility of a diplomatic signal, but said he leans more toward two other explanations being given for the apparent slowdown.

One is that rebel forces are using new trails in Honduras to move weapons from Nicaragua to El Salvador, detouring around a trail that had been under close watch and which government forces had come close to sealing off. Arms and supplies moving along new routes may escape the U.S. count and result in lower estimates of the tonnage reaching El Salvador these days, he said.

The second possibility, backed by some evidence, is that rebel forces in San Salvador may be concentrating on distributing the supplies they have already received rather than calling for new shipments.

"We're watching it closely," another official said, "but have not reached a judgment on what the apparent change in pattern means."

The United States has mobilized the high technology of its spying arsenal to keep policy makers advised of troop movements within El Salvador and the movement of equipment by air, land and sea.

"We're getting great information," one Pentagon official lamented, "but the Salvadoran military doesn't have the command and control capability to exploit it. That's why we want to train them."

The Air Force has sent AWACS (Airborne Warning and Control System) aircraft from Tinker Air Force Base in Oklahoma to monitor and report on the air traffic, much of it from Cuba, in and out of Nicaragua, Honduras and El Salvador.

The Air Force also is using its U2 spy planes, which got the photographic proof of Soviet missiles being deployed in Cuba in 1962, to photograph military activities in the region, and probably is deploying the even more sophisticated SR71 Blackbird spy plane.

The Navy's two ships in the Pacific keeping watch on Nicaragua, Honduras and El Salvador are the 3,900-ton Blakely, a Knox-class frigate commissioned in 1970, and the 3,400-ton Julius A. Furer, a Brooke-class guided missile frigate. Both carry small helicopters which can be used for close looks at small coastal vessels going in and out of Central American ports.

Although Navy spokesmen would not confirm the ships' presence or discuss surveillance activities off Nicaragua, standard techniques for such missions include homing in on and recording voice and signal communications, locating transmitting stations, logging ship movements and studying their waterlines to help determine if a ship is loaded and riding low in the water entering port and whether it exits riding high.

For such an operation, it would be standard practice for the National Security Agency to put a team of electronic experts and Spanish linguists aboard the ship. One of the team's prime objectives would be to focus electronic eavesdropping gear on rebel command posts and record the voice communications.

What frustrates many Pentagon officials is that some of the hottest information gathered by the United States is not used by the Salvadoran army fast enough to help in the war against the guerrillas.

Although spy satellites have been glamorized, military authorities say there is no substitute in a fluid situation, such as movement of enemy troops and supplies, for photographs by planes which can be sent over a hot spot at the crucial moment, sometimes at low level.

President Kennedy relied heavily on the low-level U2 photos in Cuba in 1962. President Reagan has been using aircraft photos to document Communist activities in Latin America as he seeks aid for El Salvador.