Great Britain sent troops to reinforce the garrison in Belize today in the face of a mounting confrontation with Guatemala which claims the British colony.

British army public relations officer Paul Randrup said a contingent of 15 Hercules transport planes and four VC-10 jetliners had been sent to Belize. Each Hercules can carry 250 soliders, but the planes were believed to have military equipment on board as well.

Following day-long negotiations at the Organization of American States headquarters in Washington, British and Guatemalan diplomats said they had agreed to take "prompt steps to decrese tension."

Ted Rowlands, British minister of state who participated in the discussions, is to go to Guatemala as soon as possible to talk about what measures could be taken, the joint communique said. Rowlands told reporters after the OAS negotiate. That is the important thing." Guatemalan Foreign Minister Adolfo Molina Orantes said. "I maintain my optimism." He added that Rowlands should make his trip soon because "the tension is such that we can't wait too long." He said "it is very possible" that Guatemala will react to the British garrison reinforcement by taking similar action.

In this city, there was no evidence of the increased military presence, although a number of gun emplacements had been set up around the airport.

The British government said the buildup - which included Harrier jets and a troop-carrying fright - was a response to "statements and military moves in Guatemala" which was reported to have much of its 10,000 man army to the frontier.

Belizean farmers and loggers from the western area near the Gautemalan border, frightened by reports that the Washington peace talks had broken down and that a Guatemalan invasion would come as early as this weekend, were reported fleeing toward the Mexican frontier. The number of requests for visas at the Mexican embassy had doubled to more than 100 a day.

In Guatemala. Under secretary of Foreign Realtions Alfredo Obiols warned that the British buildup could "degenerate into an armed confrontation." Housewives there began hoarding foodstuffs and prices of bread and other basic foods skyrocketed.

Guatemala also denounced the presence of the British frigate off the Belize coast as an act of "aggression," and said the buildup threatened the Washington negotiations.

Guatemala has claimed for 150 years that Belize, formerly known as British Honduras, is part of its territory and has repeatedly threatened to invade the colony if Great Britain grants it independence. Similar threats were made in 1975 and were followed by a British military buildup.

The British government statement on the current military buildup said it "in no way alters our firm resolve to seek a peaceful solution . . . Our objective remains to bring Belize to full and secure independence while taking full account of Guatemala's legitimate concern."

[State Department spokesman Hodding Carter said the British planes refueled at the U.S. Naval Air Station in Bermuda under terms of the 1941 leasing agreement with Great Britain. Carter said the United States is not involved in the dispute "except that we are interested, as are all parties, in avoiding a situation that could disrupt the peace of the hemisphere.]

A Belize radio station reported having heard of increasing troop movements of Guatemalan soliders near the Belize border, while panicky citizens on both sides were fleeing the frontier area.

Residents of the northern part of Belize were telephoning radio stations, asking them to broadcast pleas to their families in the border zones to move out of the danger area.

U.S. Consul General John Gawf said there was a "general feeling of anxiety" among the population.

"But we're not recommending that anyone leave the country," he said. "These people are put through this every few years," he added. There are about 1,000 U.S. citizens living in Belize.

Commenting on the flight of Belizeans from the border areas, Gawf said perhaps 20 per cent of the approximately 20,000 residents of the towns of Benque Viejo and San Ignacio were moving away from the frontier toward the Caribbean coast.

"I don't think you could call it an exodus," he said.