China's invasion of Vietnam Appeared to be entering a new phase tonight as Chinese troops attacked in the hills where Vietnamese defenders are guarding the shortest approach to the populous Red River Delta and Hanoi.

Radio Hanoi said that Chinese troops launched a three-pronged attack backed by tanks and artillery around the railway and road junction of Langson, which was evacuated by Vietnam earlier this week. The broadcast said Vietnamese units put a Chinese regiment and two battalions out of action in heavy fighting.

The attacks could become to biggest battle of the 12-day-old invasion if the Chinese keep pushing forward because Vietnamese defenders in the Langson area have fewer than 20 miles of hilly terrain behind them before the invaders would be in level countryside.

The vietnamese have little room to fall back there as they have chosen to do at other points where Chinese troops have come at them. Hanoi is 85 miles south of Langson.

Vitnam, which gambled throughout the first days of the invasion by not rushing major reinforcements to its defense lines, is reinforcing in the hills along the highway 1A which runs from Langson to Hanoi. According to reports here, the 308th Division, one of Vietnam's prize units, has moved toward the battle area from its positions nearer Hanoi.

Vietnam is also reportedly moving troops north from below the 17th parallel that formerly divided North and South Vietnam.

China's Vice Premier Teng Hsiaoping said this week hat one of China's goals is to destroy the myths of Vietnamese infantry prowess.

The Vietnamese, however, have been willing to give up terrain and have kept in reserve their division that gained fame fighting the French and Americans.

These tactics, coupled with China's initial caution that evidently sprang at least in part from its desire not to penetrate too deeply -- and thereby too provocatively in the eyes of Vietnam's ally the Soviet Union -- resulted in the lack of major battles in the invasion's first phase.

Despite Vietnam's reinforcements, China has the capacity to break through and advance to the delta, according to informed sources. Between 200,000 and 250,000 Chinese troops are involved in the invasion, although fewer than 10,000 have actually crossed into Vietnam, according to estimates.

China's intentions are also not clear. Although Langson clearly has become the central point of the Chinese-Vietnamese confrontation, Chinese troops could proceed as cautiously there as they have elsewhere. Nevertheless, the alignment of forces and terrain appear to give China's leaders a more promising opportunity for the kind of success they need to be able to say they have achieved the invasion's objectives.

Radio Hanoi monitored here said 1,600 Chinese were killed at Langson yesterday. No significant changes in the static battle lines were reported elsewhere along the China-Vietnam border.

In addition to bloodying Vietnam's army, China has talked of teaching Vietnam's leaders a lesson and wants Vietnam to withdraw its roughly 100,000 troops from Cambodia. According to well-informed sources, there is no evidence that Vietnam has reduced its strength in Combodia.

China has stressed that its objectives are limited, but it has been frustrated in attempts to win any of them quickly. Chinese leaders know that their risks of provoking a Soviet response increases with the length and level of fighting on the ground in Vietnam.

It seems unlikely that China has abandoned its initial caution and would send its forces sweeping down into the delta plains, but a victory in the forested hills south of Langson that raised that possibility might be seized upon by China as enough satisfaction from its invasion of its one-time ally.