The United States yesterday criticized the Vietnamese invasion of Cambodia and expressed the hope that great powers would not become directly involved in the rapidly escalating war in Indochina.

This first official American criticism of the Vietnamese war effort, tempered by a reminder of America's strong condemnation of the human rights record of Cambodia, came as Vietnam-backed Cambodian insurgents claimed to have captured two more provincial capitals in northeastern Cambodia.

The rising threat against the Cambodian government prompted a second telegram by Cambodian Foreign Minister Ieng Sary to the United Nations late yesterday requesting an urgent meeting of the U.N. Security Council. The minister said a Cambodian delegation would arrive in New York to attend the session and "enlighten" it on what he called "the savage aggression of Vietnam."

In his statement yesterday, State Department spokesman Hodding Carter said that "while the United States takes great exception to the human rights record of the government of Kampuchea [the official name of Cambodia] we, as a matter of principle, do not feel that unilateral intervention against the regime by any third power is justified."

When asked whether the Soviet Union was directly involved in the war, piloting Vietnamese fighter jets as Cambodia has claimed, the spokesman said although he had no information about the charge "we obviously are concerned that the conflict does not result in direct great power involvement."

China is Cambodia's main ally and supplies it with most of the arms and ammunition Cambodian troops are using to fight off the offensive. The threat of direct involvement of either China or the Soviet Union has plagued American officials for some time as both Cambodia and Vietnam claim that the other's sponsor is directly aiding the war efforts.

Carter also said that the United States, "as a matter of principle favors the placement of the Cambodian issue on the agenda" of the U.N. Security Council.

Last month Richard Holbrooke, assistant secretary of state for Asian affairs, said that normalization of U.S. relations with Vietnam would be greatly affected by Vietnamese actions in its year-old border war with Cambodia.

The news agency of the United National Front ofrNational Salvation, the official name of the Cambodian insurgents, broadcast claims yesterday that they had captured Stung Treng and Lomphat since the New Year. Both are provincial capitals some 120 miles northeast of Phnom Penh. They said they captured large quantities of arms and ammunition in both operations.

They also claimed to have moved within 45 miles of the capital in their attacks against government troops in eastern Kompong Cham province, capturing the provincial capital and laying siege to crucial military positions along Route 7.

While analysts here said they doubted that Kompong Cham city is in rebel hands, they confirmed that the insurgents, fighting under heavy air cover from Vietnamese jets, had crossed the Mekong River.

Since the announcement of the Cambodian National Front last month, Vietnam has claimed no role in the military offensive inside Cambodia. Western analysts, however, estimate that 100,000 Vietnamese troops are fighting in the Cambodian operation along with some 20,000 Cambodian insurgents.

Although they have fewer forces than Cambodia's 200,000 soldiers, the Vietnamese are far better equipped. The Vietnamese are fighting with captured American-made heavy artillery and tanks, as well as newly supplied Soviet arms, according to Western officials.

Radio Phnom Penh said Thursday morning that eight battlefronts had been set up by the invasion force all along the Vietnam-Cambodian border.

The radio also praised the resistance by the Cambodian troops in the face of heavy bombing by Vietnamese jets flying in front of the attacking ground forces.

The overall strategy of the offensive, begun Christmas Day, appears to be to amputate most of northeastern Cambodia for a "liberated zone" and cut off supply routes from the capital to the country's only seaport, Kompong Som, on the Gulf of Thailand.

With the siege of Kompong Cham, and the earlier capture of Kratie, the Vietnamese and insurgents have interrupted, at least, the main road and river links from the northeast to Phnom Penh.

In their southeastern push, the troops are pushing toward the rail and road routes from the capital to Kompong Som port. However, the Cambodians are midway through the construction of a new rail line from Phnom Penh to the port alongside western-lying route four.

Officials in Cambodia said last month that they were prepared to abandon territory if necessary. "Even if the Vietnamese take Phnom Penh, the struggle will always continue," said a high-ranking foreign ministry coofical. "But even if they take our cities, what do they have? Our people are in the countryside."

When the Cambodian Communists won te war in 1975 against the U.S., backed Lon Nol regime, they immediately emptied the cities and forced all the urban dwellers to work in the countryside, in agricultural cooperatives.

Because of this extreme policy, and the estimated hundreds of thousands of deaths caused by starvation, summary execution and forced labor, the insurgent Cambodian National Front claims to have won the support of many of the villagers living in the areas now contested.

In their first broadcast on Dec. 3, the insurgents decried the current government for "trampling undeefoot all traditions, customs and habits of our people."

"They [the government] have destroyed pagodas and temples of Buddhism -- an ancient state religion of Kampuchea," said the broadcast beamed over radio Hanoi. "During the past three years... a dictatorial, militarist and facist regime, matchless in history for its ferocity, has been installed in Kampuchea."

The insurgents said they would set up a "true" socialist regime and unveiled their flag: a red banner with five yellow towers representing the ancient people of Angkor set in the middle, rather than the three towers of the current regime.