China has moved major military forces toward its border with Vietnam during the past several days, according to intelligence reports reaching Washington. The troop movements are believed to be a reaction to Vietnam's drive inside Cambodia, raising concern among officials here that the Indochinese conflict could spread to major powers.

The Chinese buildup is reported to involve large numbers of troops, supplies of ammuniton and redeployment of Mig19 fighter planes and IL2, bombers to positions close to the Vietnam border. The movements were described as rapid as well as highly unusal.

U.S. officials said they do not know what China intends to do with the forces. There was some speculation that the movements may be intended as a warning to Vietnam rather than positioning for an actual strike against the Hanoi regime.

The last time the Chinese sent large numbers of troops to fight outside their territory was a 1962 clash with India. Military action against Hanoi would be more complicated, because Vietnam is a communist neighbor and is a close ally of the Soviet Union.

One governmental expert on Chinese affairs voiced doubt that a decision has been made in Peking about a tangible response to Vietnam's rapidly expanding military action in Cambodia. But he said that for Peking to take no action in the face of the Vietnamese challenge would be an embarrassment.

People's Daily, the organ of the Chinese Communist Party, charged yesterday that Vietnam has "dispatched over 100,000 regular troops to launch a full-scale offensive against Kampuchea [Cambodia] and occupy vast areas of territory under cover of Soviet aircraft, tanks and guns." The paper said this "poses an extremely serious threat to peace and stability in Southeast Asia and the Asia-Pacific region," but the commentary contained no warning of Chinese retaliation.

The Chinese Foreign Ministry also sent a "strong protest" to Vietnam charging that thousands of Vietnamese as well as ethnic Chinese are being pushed across the Chinese border. Disputes over the flow of regugees at the China-Vietnam border have been a frequent provlem between the two countries in recent months.

In a news conference with foreign correspondents, Chinese Vice Premier Teng Hsiao-ping asserted that "flagrant large-scale aggression against Kampuchea by the Vietnamese is not an isolated event but part of the global strategy of great power hegemonism." Hegemonism is the Chinese code word for Soviet maneuvering.

Teng said China has been giving the Kampucheans "all kinds of material assistance but they don't need any advisers from us because they have their own rich experience."

The Chinese have been supplying aircraft, armor, artillery and other material to Cambodia. The Soviets have been sending military aid to the Vietnamese, who are also equipped with captured U.S. equipment as well as their arms accumulated during the war that ended in 1975. Officials said here yesterday that there is no indication of a sudden increase in war supplies by either major communist power to its Indochinese ally.

Twice in recent weeks the United States has expressed official concern about Vietnam's drive in Cambodia, which is ostensibly an armed revolution by a Hanoi-backed "salvation front." However, there appears little that the United States can do to affect the fighting in Cambodia.

Morevoer, the United States appears powerlrss to head off a possible Chinese-Vietnamese clash, which might in turn touch off direct involvement by the Soviet Union with major international implications.

The United States assessment has been that China was not likely to move precipitously against Hanoi, officials said. However, they added that this was based on the expectation that Vietnam would limit its drive in Cambodia. With Hanoi "apparently engaged in a pincers movement toward Phnom Penh," an official said, earlier assumptions about Chinese counteraction may no longer be valid.

American officials doubt that China would move troops into Cambodia itself in an effort to save the Pol Pot regime. The officials said a more realistic question for the Chinese if Kampuchea crumbles is whether to maintain support of Pol Pot as an insurgent leader in the jungles, or to switch Chinese support to a more palatable group headed by Prince Norodom Sihanouk or some other Cambodian figure.