Support ships of the Soviet navy anchored for several days recently at the Vietnamese port of Da Nang, touching off new U.S. concern over a possible permanent Soviet base in Southeast Asia, according to informed official sources.

The Soviet vessels, which are reported to have docked, unloaded and left within the past week, are believed to be associated with the Soviet intelligence-gathering task force that has been stationed near the Vietnamese coast to monitor the Chinese invasion.

In other developments from the Sino-Vietnamese war area, officials said there are now clear signs that Vietnam has removed substantial combat forces from Cambodia in order to guard the area between Hanoi and the Chinese border.

There are also reports, which are given credence here, that China is not planning to withdraw from Vietnam completely, but may keep military forces on a strip of border territory it now contends is a disputed zone.

While Soviet merchant ships often called at Haiphong in the past, officials said it is rare if not unprecedented for Soviet navy vessels to put in at Vietnamese ports. Officials said the appearance of Soviet military ships was definitely a first for Da Nang, which was where the French fleet landed in 1858 to begin the era of colonial conquest and where U.S. combat forces first landed in Vietnam in 1965.

Shortly after the Chinese invasion of Vietnam several weeks ago, the United States cautioned the Soviet Union through diplomatic channels that establishment of military bases in Vietnam could be a destabilizing factor in Southeast Asia and thus affect U.S. "vital intrests," according to informed sources.

The sources said the Soviets were told that such action might cause the United States to take unspecified counteraction it would prefer not to take.

Despite the U.S. warning, there would be little surprise in official circles if China's attack increases Vietnamese dependence on the Soviet Union and leads to a greater Soviet precence in the area.

As the Soviets are said to see it, the normalization of Sino-American relations, the Sino-Japanese friendship treaty, the revised U.S. base agreement with the Philippines and other recent moves are to their disadvantage in Asia.

A greater Soviet presence in Southeast Asia, should it develop, would run counter to Chinese objectives, and in this sense the Chinese invasion of Vietnam could turn out to be a self-inflicted defeat. The reports of a partial Vietnamese withdrawal from Cambodia, however, indicate that in another sense China's invasion may be achieving its goals.

U.S. officials said at least two Vietnamese divisions and perhaps more have been withdrawn from Cambodia, where Hanoi's forces continue to be embattled by guerrilla forces of the Pol Pot regime. Some of the Vietnamese troops are reportedly being ferried to the Hanoi area by Soviet military aircraft.

The Vietnamese maintain in place most of their expeditionary forces in Cambodia. Those forces have been estimated at more than 100,000 men, but the transfer of major elements appears to be a continuing reaction to the pressure of the Chinese on the north.

Vietnam began transferring the troops from Cambodia about the time that the Chinese began their slow withdrawal from the northern Vietnam border region. The Vietnamese have charged that China does not actually intend to withdraw its troops, and Hanoi announced a general mobilization of the population a few hours after China's announcement that its forces would withdraw.

U.S. officials have noted with interest and some surprise Chinese references to disputed areas at the Sino-Vietnamese border, which had previously been considered a clearly drawn and undisputed border.

This, among other things, has given rise to reports that China may seek to keep its forces on a strip of land it claims to be its own. Such a maneuver would provide a fertile field for border clashes, which could keep the Vietnamese on edge and threaten to erupt in a renewal of larger battles.