Hanoi reported continued fierce fighting Saturday around the key border town of Langson as China launched an international propaganda offensive at the beginning of the third week of its invasion of Vietnam.

Analysts here said they could not confirm reports from some sources in Bangkok that Chinese troops have entered Langson, which controls major rail and road lines to Hanoi 85 miles away. Chinese units equipped with tanks and heavy artillery appear to have been engaged in hillside fights for several days with Vietnamese forces in the area, but analysts say they have no clear idea who is winning.

While unconfirmed rumors of an impending cease-fire continued to circulate in Peking, the Chinese began an international propaganda campaign with a 10-minute television satellite broadcast on the war and a new background document justifying the invasion.

Peking has said it wants to "punish" Vietnam for years of border provocations, although it appears even more concerned with Hanoi's close ties to the Soviet Union and Hanoi's invasion of Cambodia. Some diplomats have speculated that capture of Langson would permit the Chinese to pronounce the invasion a success and withdraw, as they have repeatedly promised to do eventually.

Saturday's satellite television broadcast, shown here, featured scenes of Vietnamese rocket damage and massive Chinese artillery bombardments that gave no hint of any plan to end the conflict soon.

In a brief battle reoort, Hanoi Radio said "three big Chinese columns were fiercely intercepted" yesterday near Langson and "initial reports say that several Chinese units were decimated." The radio disclosed, apparently for the first time, that main force Vietnamese army units had joined the fighting in the Langson area. This may please Chinese commanders reportedly eager to cripple regular Vitnamese Army units after finding only militia and regional forces at the border in the early days of the war.

The Hanoi report gave no casualty figures, unlike a Hanoi report Friday night that claimed the Chinese suffered 4,000 casualties near Langson from Tuesday to Thursday. Diplomatic sources in Peking said Chinese in some offices were told that the Vietnameses were suffering nine times as many soldiers killed as were the Chinese. Foreign analysts have judged all casualty reports so far to be greatly exaggerated.

Eye witness said Langson was almost entirely deserted by its 40,000 civilian population a week ago, and surrounding hills reportedly are being used by entrenched Vietnamese units to shell the advancing Chinese. Analysts said Chinese troops appeared to have been trying to dislodge the hillside Vietnamese positions befor advancing on the town below.

Diplomats reached in Peking said they had no confirmation of reports by Agence France-Presse and Japan's Kyodo news agency that the Chinese might be close to announcing a ceasefire. A Foreign Ministry official told one resident Peking correspondent Saturday night that there were no plans for such an announcement this weekend.

Instead, Peking television broadcast by satellite a 10-minute report on Vietnamese border provocations and what appeared to be the first official Chinese film of the thunderous Chinese assault in the first several hours of the border invasion Feb. 17. Peking sources said satellite time had been booked for the next three night for more war coverage.

Facilities for satellite transmission were built in China at the time of President Nixon's visit in 1972. Since the death of Mao Tse-tung, China has turned more and more to international television, using time on U.S. communications satellites, to present its story to the world.

Saturday night's television film began with pre-invasion scenes on the Chinese side of the border, showing bullet holes in a school, shell-damaged building, idle fields deserted by frightened Chinese farmers and a blown-up border rail bridge. The film of the opening of the invasion was an electrifying series of massive artillery flashes and tracer rounds in the predawn darkness, followed by shots of tanks moving and machine guns blazing along the line. The narrator did not say where the scenes were shot.

A seven-page document called "Background Materialc sent by mail to correspondents in Peking mostly repeatted previous Chinese charges. It said that in the six months before Feb. 15, "the Vietnamese side made military provocations against and incursions into Chinese border areas on more than 700 occasions and at 162 places, killing or wounding more than 300 Chinese border inhabitants and frontier guards."

In fighting related to the Sino-Vietnamese border war, Cambodian forces loval to the ousted pro-Peking government of Pol Pot claimed to have killed more than 750 Vietnamese soldiers and five Soviet advisors occupying Cambodia between Feb. 20 and 28.

Analysts also noted new Soviet complaints about a Chinese threat to Laos, where Hanoi appears to exert almost as much influence as it does over the new government it installed in Phnom Penh. Analysts wondered if reports of Chinese attempts to arm traditionally anti-Communist hill tribesmen in Laos might be true.

Hanoi has bitterly rejected a Chinese request for negotiations over the border dispute, a request that carried no guarantee of a withdrawal of Chinese troops before the talks began. Some diplomats wondered if Hanoi would accept a unilateral Chinese cears-fire, particularly if it was not followed by an immediate withdrawal to Chinese borders.

The Hanoi newspaper Nhan Dan said Saturday, "The Vietnamese people are determined to crush the war of aggression by the Chinese reactionaries" and called the offer of talks "another pretext for further war escalation."

"It is our consistent position that the Chinese authorities must withdraw immediately an unconditionally all their aggressor troops from Vietnam," the paper said. "This is the only way to ensure a border of friendship between the two countries and to settle all problems by peaceful negotiations."