The United States has loosened its military aid purse strings for Thailand in a modest gesture of support for the country which now faces Vietnamese tanks on its border.

U.S. officials last week approved a $6 million increase in military sales credits for Thailand as a direct response to the conquest of neighboring Cambodia by Vietnamese led rebel forces.

Moreover, Western sources expect the Thais to press for more help in improving their armed forces' fighting equipment soon, possibly when Prime Minister Kriangsak Chamanand goes to Washington on a state visit next month.

The Thais so far have not presented a big shopping list of military hardware and they are described as being more interested in speeding up the acquisition of equipment and supplies already ordered from the United States.

But sources here also think the Thai government will ask the Americans to sell several military items that have been denied in the past, in light of the new threat it sees on its border with Cambodia.

Their main fear is long-term Vietnamese subversion and support for Thai Communist insurgents located in the northern provinces on the borders with Laos and Cambodia.

The action taken last week will increase the amount of foreign military sales credits available to Thailand in the current fiscal year from $24 million to $30 million.

In terms of military hardware, it will mean a modest increase. It was granted primarily as a psychological morale booster for the Thai government.

The annual amount of military sales credits has declined slowly in the past two years and the Thai government has expressed concern that this appears to represent a lessening of American interest in Thailand's security.

"They [the Thais] regard this as a gauge of American political interest and they equate the decline with a decrease in American concern," explained one Western source.

Kriangsak has appeared anxious for a sign of renewed American support and was buoyed by President Carter's warning to Vietnam that Thailand's border should not be threatened.

The prime minister seemed to interpret Carter's remark as virtually a promise of assistance in maintaining Thailand's security.

"I fully appreciate the continued U.S. support," Kriangsak said. "We will have confidence, should anything happen to Thailand, as long as Carter is still in office."

The United States has no mutual security treaty with Thailand that would require military support. American armed forces were withdrawn from Thailand in 1976 as a result of public pressure on the parliamentary government which preceded the military coup in October of that year. That military presence, including the large air base at Utapao, had been built up to support U.S. forces then fighting in Vietnam.

However, Thailand still has a large indirect military stake in the United States.About 90 percent of its military equipment is of American origin, since the United States has trained and equipped the Thai army for nearly 30 years.

This year Thailand is buying about $100 million worth of military equipment from U.S. sources and the United States spends about $800,000 a year to train young Thai officers and technicians.

It is not clear what additional military gear the Thais will seek when Kriangsak visits Washington. In the past, the United States has refused to sell Napalm and other supplies to the government and it is believed that Kriangsak may want to renew those requests, saying a new emergency exists.

The Thai Air Force is said to be interested in obtaining more F5 fighters and large carrier planes like the American C130s as well as additional helicopters. Its navy desires surveillance aircraft needed to patrol waters of the Gulf of Thailand.

More importantly, sources said, the Thais are eager to shorten the time between order and delivery of several items. They have complained, for example, that it often takes two to three years before military tanks and other vehicles are delivered.

The American attitude so far has been that the Thai forces do not need great quantities of sophisticated modern weaponry.

"Our assessment is that they have the equipment they need and we say that they should make better use of what they have," said one source. He said the United States may urge Kriangsak to promote more economic and social development at home and to improve the country's human rights policies.

"We will stress that national security is more than beefing up defenses," the source said.