Mortar shells believed to have been fired by Vietnamese troops in Cambodia killed three Thais and a Cambodian yesterday afternoon near a refugee camp, the second fatal barrage in a week.

Later that evening, shooting erupted at the main highway crossing into Cambodia, and Thai troops responded by firing at least two mortar rounds across the border. Another eight shells reportedly struck in Thailand today but caused no casualties.

The incident increased border tensions, which began two weeks ago when troops of the Vietnamese-supported Heng Samrin government launched attacks on settlements controlled by the ousted Khmer Rouge government of Pol Pot. At least 65,000 Cambodians have fled to the Thai camps to escape the fighting.

Sunday's shelling hit an area that was later visited by three U.S. senators. Along with Assistant Secretary of State Richard Holbrooke, the senators are meeting with Thai officials and observing the refugee situation.

Many of the Cambodian refugees are armed soliders. Hanoi has upbraided Thailand for allowing soliders to "hide" in its territory, leading to speculation the Vietnamese might be preparing a cross-border strike.

[A Thai Supreme Command spokesman said Tuesday that on Wednesday its troops would begin moving newly arrived Cambodians to a more permanent camp near the town of Sa Kaeo about 35 miles from the border. The refugees would be housed in tents, he said. Thailand recently announced it would not forcibly repatritate any more Cambodians.]

On Oct. 14 a barrage of mortar shells killed five Cambodian refugees camped out about a mile inside Thailand. Thai forces did not return the fire.

The people who were killed Sunday were attending a black market about a mile from a settlement of about 80,000 Cambodian refugees. Known as Camp 007, it is operated by armed soldiers of a right wing Khmer Seri (Free Khmer) group who are harrassing Vietnamese troops in the field.

Thai officials maintain that the camp, located in an area where the border is imprecisely defined, is on Cambodian soil. But local villagers and Khmer Seri officers say it is, in fact, well inside Thailand.

Sunday's shelling came only a day after Vietnamese secretary of state for foreign affairs, Nguyen Co Thach, conducting an official visit to Bangkok, told a press conference that Vietnam will "not invade any country and . . . we will not cross the border to enter Thai territory."

Thailand has repeatedly called on Vietnam to withdraw its estimated 150,000 to 200,000 troops from Cambodia and has continued to recognize the Khmer Rouge government.

Thach said that he and Thai Prime Minister Kriangsak Chamanan recognized that great differences of opinion between Thailand and Vietnam. However, the two sides agreed to refrain from criticzing each other in the radio, press and official statements, Thach said.

Thai police officers later telephoned Bangkok editors to warn them against publishing articles that might impair relations with Vietnam. Nonetheless, Sunday's shelling was given heavy extensive coverage in the local press, with frequent references to Thach's promises of the day before.

Thai representatives in New York were instructed to file an official report on Sunday's shelling to the United Nation, a Foreign Ministry spokesman said. Thailand's ambassador in Hanoi is being recalled "for consultations." However, the spokesman stressed that the recall was not intended as a protest against the border incident.

Today the U.S. Senators and Holbrook visited two settlements of new refugees from the Khmer Rouge zones. They talked with Pol Pot soliders who said they would soon return to Cambodia to start to fight the Vietnamese.

Later, the senators, James Sasser (D-Tenn.), Max Baucus (D-Mont.) and John Danforth (R-Mo), went to Khmer Seri Camp 007. Holbrooke and U.S. Ambassador Martin Abramowitz did not accompany them, a source said, because they felt it inappropriate for State Department officers to visit a place that they believe to be on Cambodian soil. The United States has no diplomatic relations with any side in Cambodia.

The coinciding visits of Holbrooke and Thach prompted questions of whether the two would meet secretly to discuss U.S.-Vietnamese relations since the Vietnamese Embassy and official residence of the U.S. ambassador stand opposite each other on Bangkok's Wireless Road.

Vietnamese officials have said publicly that the two countries reached agreement to normalize relations late last year but that the United States reneged at the last moment. Vietnamese troops pushed into Cambodia to install the Heng Samrin government in December.

In his press conference Thach said that no decision had been taken on a meeting with Holbrooke. But it would be "useful" he said, if it benefited relations between the two countries and the situation in Southeast Asia.