The Chinese are modestly increasing their military forces near their Soviet border as well as continuing to strengthen southern border defenses in response to the Vietnamese invasion of Cambodia, government officials said last night.

U.S. officials said the first signs of Chinese movement closer to the Soviet border were the flights of an undisclosed number of Chinese Mig fighter planes. There also were signs that some ground units would be moved up soon, sources said.

So far, there has been no evidence that the Soviet Union is responding, sources said, but the possibility that the big powers could become more directly involved in the regional conflict was underscored by the State Department yesterday.

The Vietnamese-Cambodian conflict "could involve other players," State Department spokesman Hodding Carter said. "Our obvious desire is restraint," he said, adding that the U.S. government was "in direct or indirect contact with most of the players."

The optimistic forecast within the intelligence community last night was that China's military movements would turn out to be strictly in its own defense rather than a first step to aid its ally, Cambodia.

Officials taking this view said the Chinese force on the Vietnamese border consisted of fighter planes and antiaircraft weaponry for deterring any thrust into China by Hanoi. They reasoned that there were not enough Chinese troops or armor deployed for any substantial offensive.

Similarly, the Chinese redeployments toward the Soviet border were described as modest by intelligence officials, although information admittedly was sketchy.

There was fuller information on how the Vietnamese army of about 100,000 troops smashed through Cambodian forces so easily.

In one of the ironies of the seemingly never-ending conflict in Indochina, intelligence officials believe the Vietnamese adopted U.S. tactics in their thrust by first zeroing in on Cambodia's transportation system.

The United States early in the Vietnam War tried to bomb out North Vietnam's roads, bridges and ports to prevent movement of war supplies and troops to the south. That objective never was achieved.

The Vietnamese, however, quickly followed up the destruction of Cambodian air fields, ports, roads and rail lines in this campaign with an invasion of troops to hold the ground, sources said. The Cambodian government could not reinforce its military, they said.

The Vietnamese bombing, officials said, was done by U.S. planes captured by Hanoi during its takeover of South Vietnam, principally F5 fighter-bombers and A37 light bombers.

The Vietnamese, sources said, displayed impressive mobility during their preinvasion buildup and during the invasion itself, by airlifting some of their troops from the north to the Mekong Delta in the south.

Other troops moved north to south in Vietnam on the railroad that had been badly torn up during the U.S. presence in Vietnam. Hanoi made repairing that railroad a priority after taking over South Vietnam.

The Vietnamese apparently divided their invasion force of 12 to 14 divisions of from 5,000 to 10,000 troops between two main routes, one from the highlands in the north and the other from the Delta in the south, according to intelligence specialists.

Some U.S. officials predicted last night that the Vietnamese push will be followed by protracted guerrilla warfare as Cambodian units harass their latest invaders.

The Carter administration, which has decried the abuse of human rights by the Pol Pot government in Cambodia, yesterday assailed Vietnam for the latest regional conflict in Indochina.

"Vietnam is guilty of aggression," State Department spokesman Carter said yesterday. He warned that the invasion "raises the danger of wider conflict."