China and Vietnam were reported to be moving troops Thursday to the Langson area, which analysts here and in Bangkok view as the potential site of the first major battle of China's six-day-old invasion of Vietnam.

Diplomatic sources here and in Peking said the Chinese appear to be augmenting a major force assembled at Langson, about 12 miles inside Vietnam along a key rail line to Hanoi.

The warring sides provided no information on troop movements and armed engagements Thursday. Japanese news reports, however, said the Chinese were engaged in tank to-tank fighting at their most advanced point, Caobang, about 18 miles inside Vietnam, and that they were trying to break through Vietnamese defenses on the Tonkin Gulf coast.

The Vietnamese government claimed an enormous increase in Chinese casualties. It said 5,000 Chinese were killed and wounded Wednesday, more than twice as many as claimed in any single day so far, bringing the claimed total to 12,000.

Analysts here consider the figures highly inflated, but they appear to reflect a definite increase in combat at two or three key spots inside Vietnam's border.

A Soviet strategic command ship, the cruiser Admiral Senyavin, stuffed with sophisticated electronic gear, passed through the Tsushima Straits between Japan and Korea last night apparently on its way to Vietnam, sources said. In Japan, the sources suggested that Soviet Pacific fleet commander Vladimir Maslov was on board the vessel, whose equipment can link satellites, spy planes and other Soviet forces in a communications net extended thousands of miles.

Analysts predicted that the ship will join eight other Soviet vessels in surveillance activities off the coast of Vietnam, to report movements by Moscow's arch-foe, China, and one of its close allies, Vietnam. Analysts here said they had no information on reports of mobilization of Soviet forces in Mongolia and elsewhere along China's northern border.

After a few days of Chinese assaults on Vietnamese border militia forces, Hanoi has begun to move up regular army troops in substantial numbers, analysts here and in Bangkok said. Reports from inside Vietnam indicated opening artillery duels, with 130-millimeter guns that can fire 20 miles, and some troop skirmishes hear Langson, a focus for troops movements on both sides. But analysts remained uncertain whether the Chinese wanted to engage in a long battle that might not turn in their favor and could increase risks of Soviet intervention.

"Both sides are playing poker," said one analyst. "They are bringing in more troops and equipment, upping the ante and truing to lure each other out for a fight."

Analysts suggested that Chinese T59 tanks, considered about 25 years behind modern Soviet and American equipment, were moving down a northwestern flank using a hastily constructed bridge over the Red River. The Vietnamese said 140 Chinese tanks and other military vehicles have been destroyed so far, in what appears to be another exaggeration.

Bombing and strafing by Chinese Mig15s, Mig17s and F9s appears to have gone as deep as 20 miles into Vietnam, but air action has been restricted by cloudy weather in the last two days and there have been no reported air battles. The Vietnamese appear to be holding their jet intercepters in reserve for the time being.

Chinese troops participating in the invasion are said to number about 100,000, with another 100,000 to 150,000 standing ready just across the border. Some analysts estimate 10,000 to 20,000 troops are involved in the effort to take the northeastern rail line guarded by Langson. The Chinese reportedly have dug into hills surrounding Dongdang, just north of Langson. Analysts report regimental size Vietnamese units now moving north from the Hanoi area to engage the Chinese, particularly at Langson where fleeing civilians have left the city nearly deserted.

A leftist Japanese reporter in Vietnam reported a division, or about 10,000 Chinese troops, were at Laocai city, which commands the access to the the Red River and the northwestern rail line to Hanoi.

The Chinese cotinued to tell diplomats in Peking that their aims were limited in what they called their "counterattack" to end months of harassing Vietnamese border raids. The reported movement of large numbers of Vietnamese regulars to the border, however, appears to some analysts to have shortcircuited Chinese plans to get out soon.

Diplomatic sources in Bangkok insist that Chinese forces were given an order Tuesday to prepare for a general pullback, but the news leaks and the appearance of Vietnamese troops bent on making the withdrawal appear to be a rout convinced the Chinese commanders to keep fighting.

The Chinese again issued no battle report Thursday. Peking's last report, issued two days ago, said in its entirety: "Frontier forces of the Chinese People's Liberation Army in Kwangsi and Yunnan are continuing to hit back at Vietnamese troops."

The Chinese have released no official casualty reports, although Canadian and French reporters in Peking unearthed an internal Chinese document claiming 10,000 Vietnamese were killed or wounded in the first two days of the war. Western analysts consider those figures inflated also, in order to prove Vietnam has been "punished" for its border infractions and invasion of neighboring Cambodia.

The internal document said Chinese troops fought well, but had difficulty adjusting to the heat and mountainous terrain of northern Vietnam and failed to coordinate unit movements well. The Chinese apparently outnumber the Vietnamese in trained manpower.

Hanoi has only about 50,000 regular combat troops in the northern part of the country. But the Chinese army has not fought an engagement of this scale since the Korean war, while the Vietnamese have fresher experience against the Americans and the Cambodians.

China supplied Vietnam with much of its military equipment during its long war against French-and U.S.-backed governments in South Vietnam. Some of the tanks firing on the Chinese now were probably given to the Vietnamese by the Chinese. Vietnam's growing alliance with the Soviet Union and the attempt by both China and Vietnam to control Cambodia, now in Hanoi's hands, helped lead to the current conflict.

Pro-Peking insurgents are still fighting in Cambodia. Analysts here report no sign that any of the estimated 100,000 to 200,000 Vietnamese troops involved in the Cambodian conquest have been sent back to help fight the Chinese invaders.

Instead, the Vietnamese appear to have taken the Chinese at their word that the invasion would be short. They appear to have tried to avoid sacrificing their best troops and equipment. The latest move north, some analysts speculate, might be seen as a Vietnamese attempt to harass a Chinese withdrawal.

Hanoi radio is reporting outbursts of patriotic sentiment at news of the invasion, at a time when economic conditions in Vietnam are not good.

"Tens of thousands of young applications from young workers have been received offering services in the fight against the Chinese," the broadcast said, adding that some wrote their applications in blood.