Intelligence estimates that the Chinese assault on Vietnam may last for a month or longer instead of a few days are forcing a rapid reassessment of U.S. policy toward that conflict and its impact on detente with the Soviet Union, U.S. officials said yesterday.

The continuing escalation of the fighting is also causing western and communist-bloc diplomats here to ask if the Carter administration will be able to maintain its initial policy decision that the new Indochina turmoil does not engage U.S. national interests.

A protracted China-Vietnam conflict could greatly exacerbate tensions within the Carter administration itself, some diplomats feel, by forcing decisions here on matters like Western European arms sales to China and final details of a U.S.-Soviet strategic arms limitation treaty (SALT) at a time of international crisis.

While stressing that their information is sketchy and at times too contradictory to provide a sound basis for prediction, U.S. officials conceded yesterday that rapid changes over the preceding 48 hours had generated an intense new scrutiny of Chinese and Russian intentions by worried U.S. policymakers.

Chinese troops have begun digging in, refortifying their positions and establishing secure positions in the zone, 12 miles deep, that they have seized inside Vietnam, and appear to be preparing to push forward rather than pulling out, according to U.S. intelligence reports.

The absence of such Chinese preparations as they began the invasion last Saturday was cited by U.S. intelligence and military officials as reason for concluding that the Chinese thrust would be a quick, limited assault designed to "punish" Vietnam for border clashes and for invading Cambodia in January.

Estimates circulated within the administration that the Chinese would penetrate six to 12 miles and rapidly turn around after giving the Vietnamese provincial units "a bloody nose."

This assessment was buttressed by Chinese statements to diplomats in Peking that the invasion would be limited in time and scope. On Tuesday, President Carter appeared to be reflecting this assessment when he said in a major policy speech that the United States "will not get involved in conflict between Asian communist states."

Confirmed reports that the Chinese have advanced up to 18 miles at some points and are digging in, apparently to wait for regular Vietnamese divisions to move northward for a climactic confrontation, are said to be causing an urgent reassessment in the administration.

"The idea that this is just a limited border operation for dramatic punishment doesn't seem to be holding up," said one official. "It isfar more steady and deliberate than that."

Western diplomatic sources feel that the Chinese have opened a stillunfolding campaign that has as its eventual aim pulling enough Vietnamese troops out of Cambodia to topple the pro-Vietnamese regime installed there last month. But the Chinese strategy for this remains obscure.

Neither the Soviet Union nor the United States claims to have a clear idea of Chinese intentions at this time, and each is warily watching the other's response while trying to keep its own options open.

Russian options range from launching a nuclear war, in which Russian cities would also be vulnerable to Chinese atomic strikes, down to encouraging the Vietnamese to throw their best units into pursuing the Chinese back to the frontier when a withdrawal does begin.

Between those alternatives presumably lies a threshhold of Russian action in Southeast Asia that would provoke a review by the Carter administration of Western security arrangements with China as implicitly expressed by the continuing normalization of relations between Peking and Washington, and by the arms supply arrangements made by Britain and France with China.

Communist diplomats say the Russians are convinced that the Chinese invasion will be blunted, but that the resulting standoff on the border will be a cause of instability and danger that could affect the atmosphere around SALT negotiations.

Moreover, the Russians are reported deeply concerned about the timing of the Chinese assault, two weeks after the visit to Washington of Chinese Vice Premier Teng Hsaio-ping, who is being portrayed in the Russian press as sucking the Carter administration into a dangerous trap.

"Because the U.S.-Chinese relationship is still being resolved, how Washington manages that relationship during this time is a matter of grave concern to Moscow," a communist diplomat said. "And how Moscow reads U.S. intentions is certainly a matter of U.S. national interest."