The United States is seriously concerned about the increasingly bloody war between communist nations of Indochina and is taking it into account in deciding the pace of normalization of relations with Vietnam, according to a senior State Department official.

Other factors being studied in considering Washington-Hanoi relations, according to Richard Holbrooke, assistant secretary of state for East Asian and Pacific affairs, are the recent So-viet-Vietnamese treaty and the rapidly growing exodus of "boat people" from Vietnam.

Holbrooke said in an interview that "the Vietnamese have removed the major obstacle" to Washington-Hanoi diplomatic relations by dropping their demand for advance U.S. agreement to a major program of reconstruction aid.

However, he cited the current war, the Soviet treaty and the refugee flow as matters which the United States is studying "as we discuss how and when to proceed" with the process of normalization.

Holbrooke said no deadline for a decision on diplomatic ties has been set, and he would give no indication of when Washington might be ready to take action. He said confidential discussions with Vietnamese officials have continued at the United Nations, but refused to give details.

Other administration officials said there is little likelihood that President Carter will take the controversial step of establishing relations with Hanoi anytime soon. "Vietnam will have to take a back seat for a while," said an informed official source. This is particularly so, he said, because policy toward Vietnam is increasingly viewed in the context of overall U.S.-China-Soviet relations.

On the question of the Vietnam-Cambodia war, Holbrooke said "there is a serious risk" that the conflict could spread beyond the borders of Indochina and threaten the security and stability of other Southeast Asian nations, especially Thailand.

He said the United States will "not take sides" between Vietnam and Cambodia, but will seek to encourage limitation of the conflict without becoming involved itself. Washington asked the U.N. Security Council last month to take up the matter, and has backed a trip to the area by U.N. Secretary General Kurt Waldheim.

Holbrooke noted that Carter has described the Cambodia regime as the world's worst human rights offender, and said "we do not in any way wish to suggest any form of support for it." He quickly added that, at the same time the United States views a Cambodian state as a legitimate part of the independent system of states in Asia and "a generation ahead we would hope that there would be a Cambodian state."

Hanoi Radio announced Sunday the formation of a "Kampuchea National United Front for National Salvation" which is calling for the overthrow of the Phnom Penh regime. U.S. officials consider the new front a creation of Hanoi, and said its unveilling is another sign of intensified Vietnamese military and political pressure against the Cambodian foe.

The Vietnamese Communist Party newspaper Nhan Dan yesterday hailed the new front of insurgent Cambodians as a "historic event" and a "great turning point" on the road away from genocide and forced labor in the neighbor communist state.

Despite the poor state of Vietnamese relations with Peking, which is backing the incumbent Cambodian regime, China announced that a new Chinese ambassador left yesterday to take up his post in Hanoi. There has been no Chinese ambassador there for the past six months.

Vietnam's friendship treaty with the Soviet Union, signed last month in Moscow, "has caused concern about Vietnamese intentions among the nations of Southeast Asia," according to Holbrooke. "The United States shares that concern," he added.

Holbrooke's deputy, Robert B. Oakley, is on a trip to Malaysia and Thailand to consult those governments about the growing problems of refugees who are flooding out of Indochina by boat. The United States plans a major effort to seek international help for the refugees at a U.N.-sponsored conference in Geneva on the problem next week.