Cambodian guerrillas battling the Vietnamese occupation of their country have stepped up their activities in the interior, creating a new sense of vulnerability in Phnom Penh, according to foreign residents of the Cambodian capital.

In one recent incident, a vice minister of agriculture was killed and the minister of agriculture was reported wounded in an ambush in Kompong Cham Province, which borders Vietnam. One unconfirmed report said four Vietnamese advisers may also have been killed or injured while accompanying the Cambodian government delegation on a visit to a rubber plantation in the province north of the capital.

"The security situation has deteriorated badly in the sense that the war is getting closer to Phnom Penh," said one foreign relief worker who is based there.

The death of Vice Minister of Agriculture Nheng Heng in the attack three weeks ago was reported by Western aid officials and confirmed by a senior diplomat whose government maintains a mission in Phnom Penh. They said the vice minister was the highest ranking official of the Hanoi-installed Heng Samrin government in Phnom Penh to fall victim to the guerrilla war waged in Cambodia since the Vietnamese invaded 3 1/2 years ago.

No details have yet emerged about the extent of injuries reportedly suffered by the agriculture minister, Kong Samol, a U.S.-educated agronomist.

Nor was it known who carried out the ambush. Communist guerrillas of the ousted Khmer Rouge government -- driven from power by the invading Vietnamese in January 1979 after nearly four years of brutal rule -- are known to have been active in the area. But noncommunist fighters of the Khmer People's National Liberation Front also have established themselves there, according to Western diplomats in Bangkok.

An ambassador, citing reports from his country's mission in Phnom Penh, said Khmer Rouge infiltrators have become involved in work on rubber plantations in the province. The plantations, run with the help of Vietnamese and Soviet advisers, represent the devastated country's main hope for generating some foreign-exchange earnings and reducing its practically total dependence on external aid.

Besides the ambush in Kompong Cham, aid officials and diplomats report increasing attacks on the road between Phnom Penh and the port of Kompong Som, fighting in Kampot and Takeo provinces and ambushes on the railway line between the capital and the western provincial city of Battambang.

One result of the increased fighting, the sources said, was a Cambodian government decision to cut back teams deployed in the countryside by the League of Red Cross Societies.

"They know too much about what is going on in the provinces," one aid official said. Faced with demands that its teams be reduced by half, the organization has decided to pull out this month, the source said.

"I'm afraid the guerrilla war is going to get closer and be carried into Phnom Penh," he said. He added that he would not recommend remaining in the capital during the next rainy season a year from now.

Western diplomats here agree that guerrilla activity during this June-to-October rainy season has increased from last year's level, as it has every year since the Vietnamese invaded. But they see no immediate danger to Phnom Penh or significant change in the current military stalemate.

Nevertheless, the diplomats said, the guerrilla activity in the interior appears to have heightened a sense of vulnerability in the capital. Despite 3 1/2 years in power, they said, the government still depends almost entirely on Vietnamese troops for its day-to-day survival.

"The fact is there's a nasty, disciplined fighting force the Khmer Rouge only a couple days' march from Phnom Penh," one diplomat said, "and only the Vietnamese garrison is keeping them out."