The Cambodian war spilled into Thailand today for the first time at this border town.

It was a small incident in which the sounds of battle were almost drowned out by the motorcycles of the curious who came toward the border to catch a glimpse of a war that has been almost invisible to the world.

For Thailand, however. the threat that the almost two-month-old Cambodian fighting will cause new security problems has been a major concern.

Hours after about 200 soldiers crossed into Thailand, Thai officials had not determined exacty what had happened or whose side the soldiers belong to but they took no chances.

Four truckloads of Thai soldiers were brought to spread our from the usually lightly defended police station on what was once the highway linking Bangkok and Phnom Penh. Now it is a sparsely traveled road since it leads only to a barricade at the small bridge on the closed frontier.

Four tanks, lurched off the highway into the fields and officers raced back and forth in command cars ranging from jeeps to a copper Voivo directing the defense -- if an attack was indeed under way

Thai commanders also tried persuasion. A light plane flew over the border asking the intruders by loud-speaker to take their battle back to their own country.

The incident began at about dawn when fighting broke out on the Cambodian side of the border. Artillery and machine gunfire could be heard.

One Thai soldier, who, several civilians said, had been about to raise the Thai flag over the border outpost, was killed -- apparently by a stray bullet. A policeman was wounded and a school janitor who lives nearby suffered a superficial wound in the leg.

In the tiny village of Klong Nam Sai, five Thai villagers were taken across the border. For a few hours, reports circulated that a machine gun and been captured from the Thai border guards.

The intruders, meanwhile, remained in Thailand, a few hundred yards on the wrong side of the frontier, where they apparently had strayed while maneuvering against their enemies.

It was hot and quiet and soon the intruders decided to go back to Cambodia. The five villagers were released and returned to their homes unharmed. The tanks pulled into a courtyard and parked in the shade and the commanders retired to write their after-action reports.

One question remained. Were the intruders from the Khmer Rouge? Or were they from the Vietnamese Army that conquered Phnom Penh in January and has occupied most of Cambodia's towns, forcing its enemies into the forests?

At the border, villagers said the troops were Vietnamese. They had been heard speaking Vietnamese, several said. But officials were unsure.

In Bangkok, 190 miles away, the Thai supreme military command said the intruders' allegiance was unknown. A spokesman said that occasional fighting continued near Poipet, the Cambodian town across the bridge, with the Khmer Rouge apparently controlling the area.

Thai commanders' restraint on the border, where no Thai appeared to have fired a shot, was echoed by the supreme command's desire not to exaggerate the incident.

No one intended to violate Thai territory, the spokesman said. He told reporters that the intruders probably were unaware where the border ran, If so, they must have been unfamiliar with the area since the border is marked by a small stream.

Thailand has taken a neutral position on the Cambodian war. Thais and Khmers are traditionally hostile and Bangkok had no love for the barbarous Khmer Rouge government which attacked several Thai Villages in this area in the four years before it was toppled by the Vietnamese invasion.

In the most savage attack, Khmer Rouge destroyed Ban Noi Ba Rai, about a mile north of here, and reportedly killed many of the civilians. The survivors of Ban Noi Ba Rai have since been resettled in a new village farther from the border.

Yet, the Khmer Rouge were no threat to Thailand and Bangkok would vastly prefer an independent Cambodia as a buffer between itself and Vietnam's military power.

The Vietnamese-backed government of Heng Samrin has renounced Thailand for allegedly giving aid to the Khmer Rouge. It based its allegation on statements by Chinese Vice Premier Deng Xiaoping (Teng Hsiaoping) and former Cambodian leader Norodom Sihanouk who have said China is sending aid to the Khmer Rouge and that Thailand is the only available land route. Thailand has denied the charge.