The Thai government has moved military reinforcements to the Cambodian border in an apparent attempt to discourage Vietnamese-led forces from intruding into Thai territory.

The government announced Friday it was moving tanks and infantrymen to a central border position in response to intelligence reports that the Vietnamese were planning a large scale attack through that territory against Khmer Rouge soldiers still fighting in Cambodia.

That report generated alarm in Bangkok about the possibility that Thai and Vietnamese forces would clash and provoke the first major spillover into Thai territory of the six-month war in Cambodia.

Thai sources said today, however, that the border was quiet and that there appeared to be no imminent threat of a Vietnamese incursion. Other sources said the initial alarm and a subsequent quick warning to Vietnam from the United States may have been designed to deter Vietnamese forces from nearing the border.

Vietnamese-led armies and supporters of the new Cambodian government in Phnom Penh control most of Cambodian territory but remnants of the Khmer Rouge armies loyal to former premier Pol Pot hold isolated strongholds in the western and southwestern part of the country.

One Khmer Rouge unit has fought tenaciously in recent days at a point only a few miles from the Thai border town of Aranyaprathet, where a major highway connects the two countries.

The Bangkok Post, known for its access to high military officers, said Thursday that three Vietnamese regiments were preparing to make a "limited incursion" into Thai territory, near Aranyaprathet, to attack the Khmer Rouge forces from the flank. It said the Vietnamese units included tanks and missiles.

It quoted Prime Minister Kriangsak Chamanan as saying the border would be sealed off. The next day several Thai tanks and 15 trucks loaded with infantry moved to the border.

Later, however, Kriangsak said the border was calm. In the meantime, the U.S. State Department said it would be "very seriously concerned" if Vietnamese troops crossed the border and announced that the United States would be prepared to invoke the Manila Pact, an agreement under which the United States is bound to consult Thailand to meet aggression from other countries.

The result of the exchanges was the worst case of jitters Thailand has suffered since the Cambodian war began brushing up against its border in January.

Its relations with the new, Vietnamese-installed Cambodian government have been tense. Phnom Penh radio has accused Thais of assisting the Pol Pot forces by granting them refuge in southern Thailand and the Thai government on several occasions has permitted officials of the former Pol Pot government to pass through Thai territory en route to China.

In addition, Vietnamese mortar shells frequently explode on the Thai side of the border. Large sections of the long border are in dispute.

A continuing source of tension is the presence in Thailand of about 45,000 Cambodians who came across the southern part of the border. About half of them are said to be heavily armed Khmer Rouge soldiers and their dependents. They could move back into Cambodia at some future time and rejoin units of Pol Pot's army and their sanctuary on Thai soil infuriates the government in Phnom Penh.

Thailand threatened at one point to force all 45,000 of them back into Cambodia, much as it did with a similarly sized contingent at another section of the border. But after sending back about 2,500 of the refugees, Thailand suspended the repatriation for an indefinite period of time.