Thailand will request that United Nations observers be stationed along its border with Cambodia because of repeated shelling incidents, a Foreign Ministry spokesman said today.

Mortar rounds believed to have been fired from Cambodia by troops of the Vietnamese-supported Heng Samrin government have killed at least nine persons in Thai territory in the past two weeks.

[In another development, United Press International reported Cambodia's official rejection of a U.S. plan to truck emergency food supplies to starving masses disrupted by recent fighting. The Foreign Ministry "energetically rejects the plan of to all parts . . . raised by the American side," the official SPK news agency was quoted as saying.]

The Thai decision to seek a U.N. presecne came as Thai troops bused thousands of Cambodian refugees away from encampments close to the border. By this evening, about 30,000 people had been moved 40 miles into Thai territory to a new "holding center," where they apparently will be allowed to seek resettlement in third countries.

Thailand earlier had filed two protest notes with the United Nations in response to alleged encroachments. The request for observers -- Prime Minister Kriangsak Chamanon was to approve today a draft cable to U.N. headquarters in New York -- reflected increasing apprehension here that the Cambodian conflict might expand into Thailand.

Thailand is officially neutral in the Cambodian fighting, in which Vietnamese troops supporting the Heng Samrin government are trying to mop up the Khmer Rouge army of ousted premier Pol Pot. However, Thailand continues to recognize the Pol Pot government and Khmer Rouge troops have repeatedly retreated into Thailand to escape Vietnamese attacks.

Yesterday, thousands of refugees bundled up their possessions and crowded into buses that took them to a new refugee center 40 miles inside Thailand. The border encampment at Ban Khlong Wah, where an estimated 30,000 refugees had listened daily to shelling in Cambodia, was completely evacuated. Thai troops burned the grass and trees to disinfect the site, raising a column of smoke visible for miles.

Not everyone had left on the buses, however. "The soldiers, people in good health -- they're gone back to Cambodia to fight the Vietnamese," a Thai officer said. Other refugees moved by night to other spots on the Thai side of the border, apparently to help Khmer Rouge forces still in Cambodia.

At the Ban Khlong Kai Tuen camp, children boarding the buses were visibly excited -- most probably had never ridden in a modern bus. For the old and the sick, however, the journey was excruciating. One woman lay moaning in a rear a seat as dextrose solution dripped into her arm from a bottle.

In the short term, at least, the camp they moved to is no improvement over the old. Reporters visiting it found no tents or permanent shelters, just barbed wire strung around 15 acres of scrub brush. Bulldozers had torn lanes through the vegetation and refugees were setting up their makeshift plastic dwellings.