Less than three weeks before the Organization of American States convenes this year's General Assembly, the Latin American bloc has suffered another split with the severing of diplomatic relations between Guatemala and Panama.

The rupture is the fifth between Latin nations in the past eight years, and comes at a time when many OAS members are particularly anxious to meet the United States with a solid front.

Among the issues between the Latins and the United States are trade concessions, fishing rights, the Panama Canal treaty negotiations, normalization of relations with Cuba, human rights and military assistance.

Although the Latins frequently differ among themselves, they usually support each other in the face of U.S. power. Thus, Panama's struggle for control over the canal has been supported even by Latin nations that have no direct interest in the treaty income.

At the same time, while individual nations have privately deplored human-rights violations in neighboring countries, the Latins have consistently rejected attempts to bar international loans to violators. The premise is that a Latin loner will sooner or later have to depend on the support of its neighbors for its own international causes.

In the first hint of a break in Latin support for Panama's canal struggle, an editorial in Guatemala's El Grafcio newspaper last week said that there was little reason to support Panama if it could not support Guatemala's current international battle over tha annexation of the territory of Belize.

Belize - formerly British Honduras - is slightly larger than Massachusetts, with about 125,000 people. It faces the Caribbean and is bordered by Mexico on the north and Guatemala on the west.

The British granted Belize self-governing status in 1964, and have been trying to make it an independent country ever since. The roadblock has been Guatemalan claims to the territory, based on a Spanish treaty formulated at Guatemalan independence in 1821. All Guatemalan maps show Belize as a part of Guatemala - a claim supported by periodic saberrattling from Gautemala's 12,000-man armed forces.

Belizians say they do not want independence until the Guatemalan threat is removed. They feel the only thing that stands between them and Guatemalan invasion is 1,200 British forces stationed there. Their fighting force has 700 part-time troops.

Last year, Panama joined 113 other members of the United Nations in voting for a resolution supporting Belizian independence. As if that were not enough to infuriate the Guatemalans. Panamanian leader Gen. Omar Torrijos stopped in Belize two weeks ago to give a personal, raised-fist salute of support.

In an interview printed in Mexican newspapers, Torrijos then said he not only supported Belizian independence, but did not particularly care if that support angered Guatemalan President Kjell Laugerud.

It did. Calling Torrijos' statements "inexplicable and traitorous," Laugerud sent Panamanian ambassador packing.

Since then, Mexican and Colombian mediation efforts had little result.

Laugerud accused Torrijos of wanting to be "the next Fidel Castro" and suggested that a Panamanian plan to finance a new $9 million port in Belize probably would be with Cuban money.

Mexican Foreign Minister Santiago Roel Garcia called the rupture "sad, because Mexico wants a united Latin America."

Honduras and El Salvador have yet to resume relations broken in 1969 over the so-called "soccer war."

The Chilean military junta broke relations with Cuba at the time of the coup ousting Chilean leftist President Salvador Allende in 1973. In 1974, the Mexican President Luis Echeverria severed ties with Chile. Cuba is excluded from the OAS.

Last July, Venezuela closed its embassy in Montevideo after Uruguayan police allegedly broke in and arrested a Uruguayan citizen who had asylum there.

The Grenada meeting is to open June 14.