American-made F5 and A37 warplanes were roaring off the runway at Bien Hua Air Base in Vietnam again last week and heading west to drop 250-pound high explosive projectiles and antipersonnel "cluster bombs" in unacknowledged missions across the Cambodian border.

As ground battles blazed, commanders of the invading forces called in the familiar "medevac" helicopters to lift the wounded back across the border for treatment, and took solace in a "body count" of enemy dead radioed to higher headquarters.

This was no reenactment for Hollywood cameras, but the real thing, an increasingly bloody invasion by the Socialist Republic of Vietnam into the territory of Democratic Kampuchea, as those communist neighbors are officially known since the U.S. defeat.

According to American analysts who have been studying official, press and intelligence reports, the battle between them has entered a new phase since mid-June with sharply increased Vietnamese military action along lines of a conventional offensive and growing indications of countermoves to support Cambodia on the part of Chinese leaders in Peking.

In a starting reversal of roles, the Vietnamese are reported to be fighting this third Indochina was with equipment and some of the tactics of their former enemies. U.S. officials said reliable information, evidently based on radio monitorin includes reports of almost daily Vietnamese captured American-made warplanes and Soviet-supplied MIGs based at Bien Hoa and a few other airfields. Some days there have been as many as 25 missions, according to these reports.

"This use of aircraft against Democratic Kampuchea shows the ferocious, savage, greedy, aggressive, expansionist and annexationist nature of the Vietnamese.

They are no different from the U.S. imperialists and other fascists and reactionairies," charged Radio Phnom Penh in a recent broadcast detailing bombing and strafing of Cambodian territory.

As the Vietnamese have moved more in three areas, including the familiar "Parrot's Beak" and "Fishhook" regions, the conviction has grown among Washington officials that Hanoi is out to topple the current Cambodian regime and replace it with a more malleable ruling group.

Additional evidence for this belief includes abundant reports that Vietnam has trained and deployed Cambodian insurgent groups whose objective is a "revolution" against the current leadership.

The greatest uncertainty in the situation concerns China, which has the power to make any conquest of Cambodia dangerous and expensive for the Vietnamese.

On July 29, Peking announced the arrival of a Cambodian military mission "of great importance," headed by Minister of Defense Son Sen and described as "the first of its kind" since the "liberation" of Phnom Penh.

Radio Peking reported last week that the Cambodian mission met Chinese Defense Minister Hsu Hsiang-chien, despite the fact that he is currently hospitalized, and the Cambodians were reported to have conferred with "leading comrades" of the Chinese ground, naval and air force "in a profound atmosphere of mutual understanding and total agreement."

Chinese Premier Hua Kuo-feng, in his most explicit statement of alliance, was quoted as telling the Cambodian military mission, "Your struggle is a just one and a just struggle is bound to win. We support your struggle."

U.S. analysts have sharply increased their estimate of the Chinese military and civilian advisers in Cambodia from something in the hundreds to several thousand, at least, and possibly more than 10,000. The analysts say there is evidence that Chinese military adviers are stationed with artillery, air defense and naval units.

China is reported to have supplied new weaponry, including field artillery, light tanks, anti-aircraft guns, communications gear and naval patrol craft to Cambodia in recent months. The visit of the Cambodian military mission, received with so much top-level attention, suggests that additional Chinese aid may be forthcoming.

The Chinese have been making increasingly strong and high-level charges that the arch enemy, the Soviet Union, is masterminding Vietnam's actions as a means of encircling China. Defense Minister Hsu, in a lengthy article a week ago in commemoration of China's Army Day, called Vietnam "the Cuba in the East," and suggested that Moscow is behind the Vietnamese invasion of Cambodia.

Vietnam has published many reports of Cambodian attacks against Vietnamese border villages and towns, but has not acknowledged that its forces have launched major attacks into the territory of its neighbor.