The flow of Indochinese refugees has shrunk to a trickle since Vietnam tightened control over routes used by those trying to flee in boats.

According to State Department tabulations, the number of refugees arriving in other Southeast Asian countries has dropped sharply and few boats are being sighted on the South China Sea.

Part of the decline may be due to bad weather with monsoons and a series of typhoons sweeping the area. But the abrupt change, first noticeable in mid-July, is mainly attributable to new restrictions applied by Vietnam, officials said yesterday.

In May, more than 65,000 refugees came into neighboring countries, primarily Thailand and Malaysia, and into Hong Kong. That included Vietnamese leaving their country by boat and Cambodians and Laotians fleeing overland into Thailand.

The number declined to 56,890 in June, and to 27,208 in July. During the first half of August, only 3,900 departed by boat from Vietnam and about an equal number by land from Laos and Cambodia, the State Department reported.

At the Geneva conference on refugees in July, Vietnam promised to attempt to reduce the outpouring of refugees for an unstated period. The pledge was in response to protests from other Southeast Asian countries, which objected to the large numbers of refugees in United Nations-sponsored camps in their countries.

The Vietnamese government strengthened shore patrols and harbor guard posts to stem the flow of the "boat people" who often bribed their way out with payments of gold.

Vietnamese officials told foreign reporters two weeks ago that about 4,000 persons suspected of organizing escapes have been arrested. They said the government intends to continue restricting the exodus but admitted it is unable to stop every boatload attempting to leave.

The restrictions coincided with the U.S. Seventh Fleet's stepped-up operation to pick up refugees on the South China Sea. Since mid-July, the fleet has rescued 168 refugees, but only a few in recent days. And Navy reconnaissance planes are sighting fewer boats.

With more refugees being resettled permanently, the numbers residing in the camp sponsored by the United Nations High Commission for Refugees are declinging slowly. At the end of June, 368,327 were listed in camps primarily in Thailand, Malaysia, Indonesia and Hong Kong. There are about 358,000 there now and several thousand more are to be taken away in the next two weeks.

Thousands more would be leaving this fall if negotiations succeed in establishing a large temporary refugee center in the Philippines. It would be able to accommodate about 50,000 refugees.

Negotiations are under way between the U.N. high commission the Phillippines but agreement has not been reached on costs. The camp will require wells, housing and a large medical faculty, but officials said they can be quickly provided once the financing is settled.

The state Department is still pressing Vietnam to allow U.S. consular officials into Ho Chi Minh City, formally Saigon, to assist in registering Vietnamese seeking to join families living in the United States.

Vietnam so far has insisted those officials could come in only as members of a United Nations team and could not perform duties as American government representatives. Two consular officials are now in Bangkok awaiting permission to enter Ho Chi Minh City.