Cambodia charged yesterday that a suspected Vietnamese agent carried out the armed assault in Phnom Penh on three Western vistors in which British scholar and journalist Malcolm Caldwell was killed.

If so, the assault would be the latest in a number of recent Vietnamese military moves all apparently aimed at installing a pro-Hanoi government in Phnom Penh.

A Hanoi-sponsored Cambodian National United Front for National Salvation has claimed credit in recent weeks for a series of guerrilla attacks against the Pol Pot government, although apparently none has been in the capital.

Cambodia's Deputy premier Ieng Sary yesterday called the slaying of Caldwell a political assassination designed to "discredit Kampuchea," as Cambodia is now known.

Disturbed by the assassination of a writer considered sympathetic to their radical communist government, Phnom Penh officals expressed concern that it could disrupt the scheduled vist to Cambodia nex month by U.N. Secretary General Kurt Waldheim.

Waldheim and several Western journalists were invited by Ieng Sary last October to vist Cambodia in an effort to counter charges of massive human rights violations.

Along with Caldwell, Elizabeth Becker of The Washington Post and Richard Dudman of the St. Louis Post-Dispatch began their vist to Cambodia two weeks ago as the first noncommunist Western journalists to vist that country since the communist takeover from the pro-West government of Lon Nol three and one-half years ago.

Western analysts professed to be "completely surprised" at yesterday's assault, since most recent intelligence accounts suggested that the government of Premier Pol Pot was in firm control of Phnom Penh.

According to these analysts, the attack could have been carried out either by pro-Hanoi insurgents, or by local bandits, or by a dissident faction within the Pol Pot government seeking to embarras it. There was no evidence to substantiate any of these alternatives.

But a clandestine radio of the anti-government United Front for National Salvation has reported frequent incidents and uprisings throughout Cambodia. The radio, which is believed to be located inside Vietnam, became active following the establishment of the Front two months ago.

Vietnam's military push toward the eastern shore of the Mekong River and the establishment of the front suggest a long-term strategy for creating a "liberated zone" in the eastern part of Cambodia and installing a rival Communist government.

The Front, which reportedly has the allegience of an unspecified number of anti-Pol Pot insurgents, has announced a political program designed to capitalize on widespread reports of popular discontent.

Since taking power, the Pol Pot group had moved Vambodian city dwellers into the countryside, imposed a primitive barter economy and a political purge that, according to refugee accounts, involved hundreds of thousands of deaths by murder, disease or starvation.

The Front program emphasizes family life, free choice of marriage, and an eight-hour work day. All these are believed to have appeal to Cambodians whose families have been broken up by induced migrations and who have been forced into all-day labor.

Despite accounts of discontent, Cambodian troops have fought fiercely against the Vietnamese, an age-old enemy of Cambodia's Khmer people.

Faced with internal and external problems, the Cambodian govenment recently decided to expand its international links beyond China. Pol Pot's most important ally. The invitations to the journalists and to Waldheim were a part of this move to win international support and combat criticism that it has sanctioned a bloodbath.

The Vietnamese-Cambodian conflict has the potential of involving China and the Soviet Union in a protracted struggle in Indochina. The Soviet Union is allied with Vietnam and the two countries recently concluded a formal agreement of friendship and cooperation.

Analysts here believed that while the two Communist giants don not seek overt conflict in the area, they could be drawn into such a conflcit by the long-standing local animosities as well as by Hanoi's apparent aim of dominating the Indochinese peninsula.

The late Vietnamese leader, Ho Chi Minh, had advanced the idea of an indochinese federation to be created under Communist rule. Such a federation would obviously be dominated by Vietnam.

Now that Communist governments have taken power in Laos and Cambodia as well as all of Vietnam, the prospect of a Hanoi-dominated federation is profoundly disturbing to the non-Vietnamese peoples.