The United States has spotted crates of Czechoslovakian L39 jet trainers on the docks at the Bulgarian seaport of Burgas, and intelligence officials believe they are destined for Nicaragua, according to government officials.

Officials refused to discuss why they believe the planes are headed for Nicaragua.

A similar report that Soviet MiG fighters were headed for the Marxist Central American country led the Reagan administration to warn the Nicaraguans in early November that the United States would not tolerate their acquisition of such advanced jets.

But the MiG report has not been borne out. The ship that allegedly was carrying the MiGs has docked in Nicaragua, unloaded under U.S. surveillance and steamed off toward other ports.

Administration officials, however, have not relaxed their vigilance toward the acquisition of new weapons by the Nicaraguans.

U.S. officials believe that Libyan leader Muammar Qaddafi is the source of the L39s, although they say they could not be shipped through Bulgaria without the knowledge and consent of the Soviet leadership.

The L39 is a two-seat trainer plane that can be converted to a light bomber. It has a combat radius of about 300 miles and has fittings for bombs, rockets and guns.

Qaddafi, according to Pentagon officials, has piled up more Soviet weaponry than he can use. He has more planes, for example, than he has trained pilots to operate them, according to defense officials.

The suspected Libyan connection to the possible shipment of modern weapons to Nicaragua adds a new dimension to the Nicaraguan controversy.

"We're drawing the line," said an administration official when asked why the State Department and Pentagon chose to protest possible delivery of the MiG21s.

Officials declined to say whether they would regard the L39s with the same alarm that they expressed about the MiGs if the jet trainers are delivered.

Officials said there are "a few" L39s in Burgas that have been awaiting shipment for some time. They declined to say how U.S. intelligence concluded the L39s were headed for Nicaragua or how they came to suspect the source as Libya. This information is tightly held to protect intelligence sources.

One theory being advanced in the U.S. government, however, is that the Soviet Union made the MiG21 crates highly visible to U.S. reconnaissance to test the Reagan administration's reaction to their shipment to Nicaragua. One official theorized that the Soviets were deliberately focusing administration concern on the MiG21s to make it easier to ship the L39s without becoming embroiled in a similar controversy.

Since the L39s, along with crates of helicopters, have not been shipped and it would take some time to get them to Nicaragua if they are put on ships, the Reagan administration may opt to remain quiet about this latest batch of modern weaponry in hopes that it will not leave the dock, given the line that has been drawn.

Although the State Department and Pentagon have stressed that the MiG21s might upset the military balance in the region if Nicaragua should get them, several military officials with combat experience said the Soviet Hind helicopters would be much more lethal in Managua's war against the contras in the country. The Pentagon has said that Nicaragua appears to be getting 12 Hind Mi24 helicopter gunships, which can lay down a deadly fire of bullets and rockets. The helicopters are armor-plated against ground fire.

The Pentagon has said that Nicaragua has no need for MiG21s. This plane, considered a home defense interceptor of invading bombers when employed by the Soviet air force, is far beyond anything Nicaragua needs for its defense against countries in the region, officials contend. The MiG21, if sent against American carrier aircraft that might be based off Nicaragua, would be far outclassed by the Navy F14 fighter, experts say.

The L39 would be useful to Nicaragua because of its capability of being both a trainer and a light attack plane. The Honduran air force has some U.S.-built A37 Dragonflies which have limited bombing capability.