The Thai Army moved reinforcements to the Cambodian border today after 82mm mortar shells -- believed to have been fired by Vietnamese forces inside Cambodia -- killed at least four refugees and wounded 12 in a camp less than a mile inside Thailand.

Eight shells landed on Thai territory, the Thai Supreme Command said. The explosions touched off panic among the estimated 30,000 Cambodians who have crossed into Thailand since Wednesday when troops for the Vietnamese-backed Heng Samrin government begain an attack on a major Khmer Rouge stronghold in the Phnom Malia hills.

The shelling, coupled with the Thai reinforcement at the border, raised fears of a clash between Thai and Cambodian forces.

The shelling came as a Vietnamese newspaper accused Thailand of allowing Khmer Rouge soldiers to "hide" in Thai refugee camps.Thai officials and refugee leaders maintain that all the refugees are civilian. However, many are young men who, although now unarmed, appear to be soliders.

Thai soliders, who have been guarding the refugees near the well-marked border, used megaphones to plead for calm and promised that they would provide protection against attacks. Nonetheless, thousands of refugees packed up their belongings from beneath trees and tall grass and moved about half a mile deeper into Thailand, Thai military officials said.

Thai Air Force reconnaissance planes were dispatched to the scene, and Thai artillery reportedly was moved into the refugee camp. The Supreme Command said Trai troops did not return the fire.

Despite the quick movement of Thai reinforcements, officials here were obviously anxious to avoid escalating tension in the area.

"It must have been an accident," a Thai spokesman said, referring to today's shelling. Khmer Rouge and Vietnamese forces are fighting each other just across the border and "only one time have shells fallen into Thailand."

One military specialits here noted the shells might have come accidentally from Khmer Rouge forces in the area. The Pol Pot troops are known to use 82mm mortars, which require a highly trained crew to be fired with accuracy, he said.

Since the Vietnamese Army moved up to the Thai border last spring -- it advanced west after driving the Khmer Rouge government of Pol Pot from phnom Penh in January -- the Thais generally have avoided responding to minor provocation. A week ago several 105mm howitzer shells landed in Thai territory, but the fire was not returned.

However, by bringing in their own artillery and antitank weapons -- Thai soldiers at the refugee camp last week were seen carrying portable antitank rockets -- Thailand could be signaling to the Vietnamese that their troops should not attempt a cross-border strike at the Khmer Rouge camp.

On Saturday artillery and small arms fire continued to be heard from just across the border. Several thousand Khmer Rouge troops are believed to be holding out in bunkers and trenches.

Meanwhile, news services reported the following :

Cargo planes ferried 55 tons of rice, fuel and other relief supplies to Phnom Penh for Cambodia's starving people, but organizers of the emergency international airlift said they feared a famine might not be averted. e

The Cambodian government agreed to the relief program Saturday, and it began even though formal agreement has been held up over the government's insistence that no food be given to Pol Pot's loyalists.

As the operation got under way, a spokesman for UNICEF, one of the two agencies coordinating the relief effort, estimated it would take 165,000 tons of food during the next six months, or more than 900 tons a day, to stabilize conditions in Cambodia.

The official said more than 10,000 tons of food are scheduled to be shipped to Cambodia by the end of the month.

Thirty nations have pledged more than $110 million to the relief effort, coordinated by the Red Cross and UNICEF.