Chinese and Vietnamese forces are batting with artillery, heavy tanks and infantry for a key city on Vietnam's northern rail and road lines leading to Hanoi, analysts here and in Bangkok said Friday.

The battle near the city of Langson, 12 miles inside Vietnamese territory, was going on as Chinese forces continued to probe Vietnamese defense elsewhere along a 500-mile front. Chinese forces, which earlier captured another key road junction, the northwestern town of Laocai, have now moved six miles farther down the Red River toward Hanoi, still about 140 miles away. Vietnamese forces have fallen back to a new defense line, apparently at Pholu, diplomatic sources said.

Bangkok analysts said the Vietnamese troops fighting the Chinese at Langson were from regional and not main force army units. But unattached regular army regiments based south of Hanoi were apparently moving up north, while three regular army divisions remained in protective positions just north of Hanoi and Haiphong, Vietnam's main port.

The Bangkok analysts also reported that at least four AN22 transport aircraft flew to Vietnam from the Soviet Union, reportedly making refueling stops in Iraq and India. The analysts speculated that the 45-ton capacity planes were carrying urgently needed spare parts and equipment such as antitank and antiaircraft weapons.

There had been reports of at least one Soviet airlift to Vietnam during Hanoi's invasion of Cambodia last year. Regular sea shipments of military spare parts and other equipment also occurred during the Cambodia fight, but this appeared to be the first airlift connected with the Chinese invasion.

Sources here and in Bangkok said they could not confirm reports by Thai intelligence sources quoted by the Associated Press that Chinese aircraft had bombed a supply unit near Haiphong bringing Soviet military equipment to the border. The sources said their information indicated that Chinese planes had not ventured deeper than about 20 miles into Vietnamese territory. The planes reportedly are giving close air support to Chinese infantry. A Peking official quoted by Japan's Kyodo news service denied the report. These sources said there have so far been no tactical air engagements by Chinese and Vietnamese jets and no deep penetration by Chinese bombers.

[Officials in Washington cautioned the press and public yesterday against "hot stories" from the war zone, saying that many are exaggerated or erroneous. See story, Page A18].

The Soviet airlift and the movement of a cruiser with the Soviet Pacific fleet commander on board to join a small group of Soviet ships in the South China Sea provided the most immediate signs of Soviet concern over the attack on its close ally, Vietnam. Analysts here and in Bangkok, however, said they did not know of any significant mobilization or movement of Soviet troops on China's northern border, where Soviet intervention could turn the so far limited Sino-Vietnamese clash into a major incident.

[U.S. Army intelligence sources in Tokyo denied all reports of Soviet military mobilization during the past week, Washington Post correspondent William Chapman reported from Tokyo.]

On the diplomatic front, the Soviet Embassy in Peking cancelled the annual Soviet Armed Forces Day reception as a sign of "reprobation at the Chinese agression against Vietnam," according to an embassy spokesman.

Analysts here and in Bangkok said some of the Vietnamese Army reinforcements moving northward may be part of the 20,000 Vietnamese troops stationed in Laos. Analysts said they could not confirm reports that troops from the large Vietnamese force in Cambodia are also being called back to fight the Chinese.

The Vietnamese forces helped topple a pro-Peking government in Cambodia early in January, but battalionsized units loyal to that ousted government have now regrouped and are making some gains against the Vietnamese in the northwest of Cambodia. Analysts noted that any effort to move Vietnamese forces from Cambodia to the Chinese border would take at least four or five days, given Vietnam's shortage of transport aircraft.

Analysts speculated that Chinese commanders were delighted to see large numbers of Vietnamese troops and equipment moving toward the border, since the Chinese goal has been to weaken Hanoi's army and discourage further raids across China's border and further action in Cambodia.

"Their objectives are blood, revenge and humiliation," said one analyst, who noted that up until now the Chinese have only been able to maul scattered border militia. One analyst suggested that a general order to prepare to withdraw radioed to Chinese troops earlier in the week might have been a calculated attempt to draw the Vietnamese toward the border in the vain hope of harassing a retreating Chinese army.

The Chinese now appear to have captured three major border towns, Mongcai, Caobang, and Laocai and may also have a fourth, Laichau. Some analysts said they expected Langson would fall soon, if it had not already, but they had no confirmation that Chinese troops had entered the city. Most of the 46,000 Vietnamese inhabitants of Langson have already been evacuated, according to foreign correspondents visiting the area.

The deepest Chinese penetration appears to be at Caobang, about 18 miles inside Vietnam. Analysts said most of the invading Chinese force is no more than one to three miles inside Vietnam, apparently destroying border fortifications and wiping out resistance.

Although the Chinese have committed about 250,000 troops to the operation, most of that force is apparently still in China. Military analysts calculate that a Chinese move deeper into Vietnam along the whole front would take many more troops than are currently committed to the operation. Efforts to move in columns down just the two main roads to Hanoi, from Laocai and Langson, would expose the Chinese to effective counterattacks from the Vietnamese.

The Karst Mountains terrain of northern Vietnam provides excellent cover for ambushes. Some analysts predict that the Chinese will not move much deeper, but instead will wait for the Vietnamese to engage them in their dug-in positions.

Analysts acknowledged that the longer the Chinese remain in Vietnam, the greater is the chance of Soviet involvement.

The Chinese have given no official statistics on the progress of their forces in Vietnam, which they say are conducting a "counterattack." But the official New China News Agency Friday night provided the first Chinese accounts of fighting inside Vietnam.

One item told of a Chinese arms bearer who, empty-handed, captured an armed Vietnamese soldier. Another extolled deputy squad leader Li Cheng-wen who "gave his young life to clear the way of advance." Li stood and held a package of lighted explosives up against a Vietnamese pillbox and even "found time to wave his waiting comrades to move onward" before the blast killed both him and the Vietnamese inside.

The official Vietnam News Agency, which has been giving casualty figures considered by analysts here to be greatly inflated, reported 1,000 more Chinese casualties Thursday. This raised the claimed total of Chinese killed and wounded in the week-old war to 13,000. The Vietnamese indicated that much of the action was around Langson and a town just north of it, Dongdang.