Vietnamese troops appear to have intensified attacks on a major Khmer Rouge stronghold close to the Thai border in recent weeks. Phnom Penh radio, meanwhile, has charged Thai forces with shelling the Cambodian area and sending over observation planes.

The fighting appears to involve small Vietnamese ground units backed by mortars and artillery. According to Thai military sources, action has centered around a bridge leading into the Phnom Malai hills in western Cambodia.

Several thousand Khmer Rouge soldiers are believed to have built tank traps and bunkers in the hills. Once access is secured, Thai sources have said, the Vietnamese will use tanks and artillery in an offensive against the Khmer Rouge positions.

Such an attack could prompt the Cambodian guerrillas to retreat into Thailand, raising the possibility that the Vietnamese would pursue them and confront the Thai Army. Hanoi's forces intruded briefly into Thailand three weeks ago at a point about 25 miles northeast of Phnom Malai.

However, diplomats in Bangkok cautioned that Thai sources repeatedly has erupted intermittently in Phnom Malai ever since Khmer Rouge guerrillas retreated there in the face of Hanoi's invasion of Cambodia last year.

"There's obviously something going on," commented one Western diplomat with access to intelligence reports on the border situation. He noted recent reports of increased casualties appearing at Khmer Rouge hospitals close to the frontier.

Phnom Penh radio said Thai observation planes and helicopters had overflown Cambodian territory on three successive days earlier this month. It said that after two planes passed over Phnom Malai on July 7, Thai forces fired a rocket and mortar barrage into the hills.

The new fighting came as the Khmer Rouge claimed new successes in their guerrilla war elsewhere in Cambodia and as tension heightened on Vietnam's border with China.

In recent weeks, the Khmer Rouge have reported derailing two trains which they said were carrying Vietnamese troops. Guerrillas killed 33 Vietnamese on the first train in the June 28 attack in Battambang Province, and 119 in the second ambush in Pursat Province eight days later, according to Khmer Rouge radio.

In Hanoi, Vietnam News Agency reported that on July 10, Chinese forces fired 200 mortar shells into Ha Tuyen Province, killing or wounding a number of civilians and destroying houses and crops.

China had earlier charged that Vietnamese forces fired on a commune and kidnaped four fishermen. During May and June, Vietnamese had committed 114 armed provocations along the border, the Chinese government alleged.

Meanwhile, foreign relief agencies and the Thai government appear to have agreed here today that food aid for Cambodia should continue to be distributed at the Thai border as well as inside the famine-stricken country.

Earlier reports had said the International Red Cross opposed further shipments to border areas controlled by guerrillas fighting the Vietnamese-supported government in Phnom Penh because much of the food was going to soldiers.