Thailand, which served as the launching pad for much of the U.S. bombing of Vietnam, announced today that it was establishing normal relations with the Communist government in Hanoi.

The step, combined with a number of other recent diplomatic developments in Southeast Asia, was a clear indication of newly improved understanding among the three Communist states of Indochina and the five non-Communist members of the Association of Southeast Asian Nations (ASEAN).

The announcement by Foreign Minister Upadit Pachariyangkun said the agreement with Hanoi was discussed during meetings with Vietnamese representatives at the U.N. General Assembly in September. It was completed by the Thai and Vietnamese ambassadors to Laos.

The agreement to establish diplomatic relations had actually been reached during a meeting of Thai and Vietnamese foreign ministers held in August 1976, but developments were frozen after Thailand's blooldy coup d'etat the following month. Today's announcement finally implements the earlier accord.

Nevertheless, it still could signal a breakthrough in relations among the Communist and non-Communist nations of Southeast Asia, a relationship that has been strained since the war in Indochina ended more than two.

"We will try to support and revive relations between Thailand and Cambodia and Laos." Upadit said in reply to a question at a news conference.

Thailand and Cambodia do not have diplomatic ties. While Laos and Thailand maintain embassies in each other's capitals, the relationship has frequently been chilly in the last two years.

At independence day celebrations in Vienttiane this morning, Loatian Premier Kaysone Phomvihan said country wanted to improve its relations with Thailand as well as with the United State.

The U.S. embassy has continued to function in Vientiane since the Communist takeover but it is not headed by an ambassador.

Upadit said that Thailand and Vietnam would exchange ambassadors "as soon as possible." He said the move world benefit both countries economically. From Thailand's viewpoint, he noted, talks were expected to begin this month on rights for Thai international flights to pass over Vietnamese territory.

From Hanoi's perspective, the minister said, improved relations would lead to a resumption of U.N. supported development projects along the Mekong River, which passes through Vietnam and Cambodia and forms part of the border between Thailand and Laos.

Some observers here believe that economic hardships here believe that economic hardships were the paramount factor in Hanoi's decision to implement the diplomatic relationship, Vietnam and Laos have been suffering from severe markedly this year. The Thai crop has also declined, bit as a traditional exporter of huge quantities of rice. Thailand's domestic consumption has not been affected.

Whether Vietnam anticipates buying rice from Thailand is not known, but Thai sources have indicated that this country is prepared to begin exports of salt, sugar, textiles and several other consumer itmes to all three Indochinese states.

Laos, which is landlocked, has long relied on Thailand as a transit corridor for imports of virtually all its necessities, including food and fuel. The Thais have periodically cut off this flow in order to pressure the Communist government in Vietnam.

Now that Thailand and Vietnam have warned toward each other, it is likely that an improvement will develop between this country and Laos, which has a "little brother" relationship with Vietnam.

Cambodia, sealed off from virtually all the world and marred by an image of brutality, also seems to have been begun to emerge. This week, for example, the president of Burma, Ne Win, visited the Cambodian capital, Phnom Penh. The Cambodians have also announced that the foreign minister of Malaysia, a prosperous non-Communist ASEAN member, is to visit.

The Cambodians continue to have close ties to China, unlike Vietnam and Laos, which are linked to the Soviet Union. The Chinese are known to have been displeased with Cambodia's radical domestic and international policies.

During a recent visit to Peking, Cambodian premier Pol Pot was reportedly urged by the Chinese to tone down some of his government's more extreme policies, including the frequently bitter border struggle with Thailand.

The Thais have launched diplomatic initiatives that had been shelved during the year-long rule of former premier Thanin Kraivichien, a fervent anti-Communist.

Thanin's successor, Gen. Kriangsak Chomanan, reportedly wrote personally to Vietnamese Prime Minister Phem Van Dong, declaring his government's desire for diplomatic relations.

This step by the Thai strongman is seen as an indicator of Thailand's new pragmatism in regional policies. Likewise, Vietnam's agreement may be judged not just in light of its economic difficulties, but as a clear sign that is wants to begin an era of peace in Southeast Asia.