The Soviet Union has just completed an airlift of men and military supplies to Vietnam, the first such concentrated resupply since the fall of Saigon three years ago.

The airlift involved more than two dozen Soviet flights to Hanoi's Noibai airport with stops in Pakistan and India, according to U.S. officials. The flurry of unusual and unscheduled flights began in mid-August and ended several days ago.

Most of the aircraft were described as civilian transports believed to have carried Soviet technicians to replace the Chinese technicans who pulled out this summer as Sino-Vietnamese tension mounted. But some flights were of military aircraft apparently taking arms and ammunition for the continuing Vietnamese offensive against Cambodia, sources said.

Although the military and economic impact is expected to be modest, the Soviet demonstration of support for Hanoi is likely to increase Chinese apprehensions about encirclement and thus add new fuel to the Sino-Soviet conflict in Asia.

As China continues its sealift of military supplies for the Cambodian forces and the Soviet Union steps up its support for the opposing Vietnamese, the "third Indochina war," as the Cambodian-Vietnamese battle is sometimes called, increasingly takes on a proxy war dimension.

A top-level Cambodian military delegation was given high visibility and red-carpet treatment on a visit to Peking a month ago, giving rise to expectations of increased Chinese aid. Chinese Premier Hua Kuo-feng was quoted as telling the Cambodians, "We support your struggle."

Earlier this week, Hanoi Radio braodcast a statement that is said had been made public by the Soviet Politburo, with the approval of President Leonid I. Brezhnev, affirming Moscow's "unshakable solidarity" with Vietnam and attacking Chinese aid to Cambodia and "the gross chauvinist pressure" being exerted by China in the still-growing conflict over Chinese resident of Vietnam.

Sino-Vietnamese talks in Hanoi have broken off, and tension has risen along the common border of the countries. Vietnam has been calling on its people to "mobilize for mass resistance . . . to the aggressive and expansionist threats we face today" and giving other indications that a shooting war with its Chinese neighbor is not outside the realm of possibility.

During the Indochina war against the United States and U.S.-backed South Vietnamese forces, most of Moscow's aid to North Vietnam was shipped overland through China or sent to Haiphong via Soviet ships. With transit of China now out of the question and an airlift around the rim of China a cumbersome affair, U.S. analysts are keeping a close watch for signs of Russians shipping bound for Vietnam.

One conjecture is that Moscow's airlift was a quick shot in the arm for its major Asian ally. A more extensive aid program would probably be mounted by sea, in this view.

The number of Soviet technicians and other personnel involved in the recent flights is believed to have been well into the hundreds. As a result, U.S. estimates of Soviet military and civilian personnel in Vietnam have been raised from 2,000-to-3,000 to 3,000 to 4,000 though figures are considered imprecise.

Peking announced in early July that it was ending technical and economic aid to Vietnam in retaliation for what it called "obdurate stepping up" of anti-Chinese activities. Reports at the time said that at last 72 Chinese aid projects in Vietnam had been canceled and Chinese technicians withdrawn.