A serious strain reportedly has developed in Soviet-Cuban relations over what President Fidel Castro views as Moscow's weak and indecisive response to Reagan administration pressures on Nicaragua.

Eastern European sources said the Cuban leader felt both frustrated and annoyed by the late Soviet president Konstantin Chernenko's conciliatory approach to the United States. Moscow has hardly reacted to a series of public pronouncements aimed at the leftist government in Managua, including a news conference comment by President Reagan last month that in effect said he was seeking the removal of the Sandinistas from office.

Castro failed to attend Chernenko's funeral this month, presumably to register his displeasure with Soviet policy toward Nicaragua. Castro also did not sign the book of condolences at the Soviet Embassy in Havana. The Cuban leader had attended the funerals of Chernenko's two predecessors, Yuri Andropov and Leonid Brezhnev. Castro's brother, Raul, represented Cuba at Chernenko's funeral.

Gorbachev met Wednesday with Raul Castro for talks which, according to an official communique, passed in the spirit of "fraternal friendship, cordiality and full mutual understanding." Castro, who is Cuba's vice president and defense minister, had conferred earlier with Marshal Sergei Sokolov, the Soviet defense minister, and other senior officials here.

Sources here said that Fidel Castro had become profoundly annoyed with Chernenko last March when the Soviet leader refused to allow a Soviet naval flotilla to approach Nicaraguan waters. The flotilla was on its way to Nicaragua when a Soviet tanker was severely damaged by a mine at the entrance to Nicaragua's Pacific harbor of Puerto Sandino.

According to the sources, Castro was turned down when he urged Moscow to have the flotilla proceed to Nicaragua to signal Soviet military backing for the Sandinista government.

In another incident, U.S. television networks quoted Washington sources last November as reporting that Soviet MiG21 jet fighters were en route to Nicaragua. But Moscow sent no such jets, limiting its aid to several tanks and helicopter gunships.

Another possible source of Soviet-Cuban friction, reported from Havana by Reuter, involves sugar exports. Soviet demands that Cuba fulfill its commitments to supply sugar to Soviet Bloc nations, the news service reported, have forced Havana to spend $100 million buying sugar on the world market.

The Soviets have been extremely reluctant to project their power so far away from their territory. Instead, they apparently have decided to try to relieve the U.S. pressure on Managua by signaling to the Reagan administration that they can inflict damage on U.S. interests closer to Soviet borders. Reports in well-informed Soviet circles recently have suggested that the new Soviet leadership was now considering unspecified actions against Pakistan, an American ally, in case of a direct U.S. military move against the Sandinistas.

This view was reinforced by a report of the meeting between Gorbachev and Pakistani President Mohammed Zia ul-Haq, who was here for the funeral of Chernenko. According to the official news agency Tass, Gorbachev sharply criticized Pakistan for supporting "aggressive actions" mounted on its territory against Afghanistan, a Soviet ally, and warned Zia that his policy "cannot but affect in the most negative way Soviet-Pakistani relations."

There is little doubt that Moscow would become militarily engaged if there was a threat to Cuba, which is the most important physical and political bridgehead for Soviet influence in Latin America.

But in general terms, the Soviets tend to see the rest of Latin America much as Americans see Eastern Europe. Moscow recognizes that the United States is the dominant power in Latin America. But the Soviets also realize that any weakening of U.S. political and economic influence in the region becomes an important factor in the global competition between the superpowers. Anti-Americanism in Latin America thus is as important for Moscow as anti-Soviet sentiment in Eastern Europe is for Washington.

In practical terms, however, a hard-pressed Soviet Union is reluctant to increase its economic and other commitments in the area. The distances involved and the accompanying logistical difficulties make it highly unlikely that Moscow would undertake direct military commitments in any part of Latin America except Cuba.

Castro's commitment to the Sandinista government, according to western diplomats here, seems to be far more ideological than Moscow's. Chernenko's decision to have the Soviet flotilla return home instead of proceeding to Nicaragua was apparently taken by Castro as a sign that Moscow's support for the Sandinistas in case of direct U.S. intervention would be largely verbal.

The dispute has surfaced here in an oblique way. The latest issue of Kommunist, the most authoritative theoretical journal of the Soviet Communist Party, contains an unusually warm and laudatory article about Che Guevara, the Argentine revolutionary who was one of Castro's top associates and later was killed while trying to organize a leftist revolution in Bolivia.

But the Kommunist article described Guevara as a "revolutionary romantic" and quoted pointedly from Guevara's letter to Castro in which Guevara said he was going to Bolivia, "where I can do something which is denied to you by virtue of your responsible post as leader of Cuba."

Implicit in the article is a message to Castro that ideological consistency and revolutionary fervor are often in conflict with a state's national interests. One could construe this as a Soviet explanation for Moscow's policy of restrained support for Nicaragua.

The article was approved for publication last month and thus presumably reflected the thinking of the Chernenko government.

The hints of Moscow linking Nicaragua to Pakistan and its support of Moslem rebels fighting the Soviet-supported Afghan government of Babrak Karmal appeared to place both issues in a new context. It is possible, some observers here speculate, that the new Soviet leadership plans to get tough with Zia in an effort to stop Afghan rebel activities on Pakistani soil.

According to this line of thinking, Moscow's pressure on Zia may have been coupled with an effort to force Washington to halt its direct military pressure on the Sandinistas.