China launched a major armed assault against Vietnam Saturday morning along the length of their mutual border, with Chinese troops and tanks, under air support, reportedly plunging at least six miles into Vietnamese territory.

Peking announced that its frontier troops had "been forced to rise in counterattack" to end months of Vietnamese border harassment and did not intend to occupy Vietnamese soil. But the coordinated assault appears to greatly increase the chances of an all-out Sino-Vietnamese war which could have global effects if Vietnam's close ally, the Soviet Union, retaliates against China. Hanoi has asked for aid from Moscow.

Hanoi Radio said Chinese infantry supported by artillery, tanks and aircraft, had struck four Vietnamese border provinces, Quangninh, Lang Son, Caobang and Hoanglienson. The radio said Vietnamese forces had killed 250 Chinese troops and destroyed many tanks and weapons in Hoanglienson fighting. A Chinese news bulletin said battles were continuing past midnight.

Western analysts have estimated that recent reinforcements have brought to at least 100,000 the number of Chinese troops in the border area. The Vietnamese, who have at least that many troops tied down in a guerrilla war in Cambodia, are said to have fewer soldiers on their Chinese border, but they are dug in and well equipped.

"The Chinese frontier troops are fully justified to rise in counterattack when they are driven beyond forbearance," the official New China News Agency said in a statement authorized by the Chinese government. "We do not want a single inch of Vietnamese territory, but neither will we tolerate wanton incursions into Chinese territory."

The words echoed Chinese statements during its 1962 invasion of northeastern India, which had been incited by Indian moves into a disputed mountainous border area. In that instance, after marching deep into Indian territory, Peking declared a cease-fire and unilaterally returned to its border, apparently confident the Indians would attempt no further border incursions.

Unlike the Indians, however, the Vietnamese have a strong ally in the Soviet Union, with nearly a million troops stationed near China's northern border. There have been reports in recent weeks of military buildups on both sides of the Sino-Soviet border in China's Sinkiang region and some evacuation of Chinese civilians in that area.

The Vietnamese government, in a statement issued Saturday night by the official Vietnam News Agency, called on "the Soviet Union, fraternal socialist countries" and other friendly powers to "support and defend Vietnam."

There was no immediate official Soviet reaction to the Chinese attack on Vietnam. The Soviet news agency Tass distributed the Vietnamese government statement. Tass also reported that the Chinese attacked in 14 different directions and that Chinese forces penetrated at least four miles into Lang Son Province. It said 13 Chinese tanks were destroyed in fighting near Batsat in the Hoanglienson area.

The Soviet Union and Vietnam signed a friendship and cooperation agreement last November that provides for mutual consulations in the event of attack, or threat of attack, on either signatory. Section 6 of the treaty provides for the parties to "immediately initiate bilateral consultations to eliminate this danger and take appropriate measures to maintain their countries' peace and security."

A sketchy Vietnamese battle report said "Chinese motorized infantry, armor and artillery divisions with air support launched a massive attack on the whole Vietnam-China border.

"Chinese motorized infantry supported by artillery attacked Muongkhuoung district town and opened wanton artillery fire on the town, power plant and the railway station." There were other reports of Chinese artillery attacks on Vietnamese towns but no estimates of Vietnamese casualties.

The official Chinese accounts Saturday carried almost no information about the fighting.

The Chinese news agency, in one of many dispatches on the border situation that have dominated the official Peking press in recent days, said Saturday that people in the border town of Hekou "saw with their own eyes three Soviet advisors in a Vietnamese trench gesticulating toward the Chinese side" late last month."This incident serves to prove that it is the Soviet instigation and support that have made the Vietnamese authorities run rampant," the Chinese agency said.

In justifying the attack, the government-authorized statement issued by the Chinese agency Saturday night said the Vietnamese authorities "have concentrated massive armed forces along the Sino-Vietnamese border and repeatedly encroached on China's territory. They have flagrantly laid mines and built defence works on Chinese territory, willfully opened up with rifles and guns to destroy Chinese villages and kill Chinese soldiers and civilians, plundered Chinese properties and attacked Chinese trains, thus causing grave incidents of bloodshed.

"In the past six months alone, the Vietnamese have made armed provocations on more than 700 occasions and killed or wounded more than 300 Chinese frontier guards and inhabitants.

"By such rampant acts of aggression the Vietnamese authorities have meant to provoke military conflicts and heighten tension along China's southern border and disrupt China's socialist modernization program."

In the last year, as Vietnam's war against China's ally, Cambodia, grew more serious and China cut off all aid to its former socialist ally, Hanoi complained bitterly of similar scattered border raids and violence by Chinese troops. The Vietnamese government statement Saturday night said that "by invading Vietnam, the Chinese rulers have totally revealed their expansionism, big-power hegemonism and reactionary policy."

China's attack came as Vietnamese Premier Pham Van Dong was in the Cambodian capital of Phnom Penh for talks with the pro-Hanoi insurgents who have taken over the government of Cambodia with the assistance of Vietnamese troops. Foreign analysts speculated that the Vietnamese premier was negotiating a treaty with the new Cambodian president, Heng Samrin, to ensure continued Vietnamese control over the new government and Vietnamese armed participation in the guerrilla war against remnants of the Peking-backed Pot Pot government.

In Hanoi, Vice Foreign Minister Nguyen Ko Thac told foreign ambassadors earlier Saturday that Chinese forces have occupied nearly all frontier posts on the Vietnamese side of the border. He said Vietnam intended to raise the matter at the U.N. Security Council.

The Chinese agency, meanwhile, reported that Peking's U.N. representative Chen Shu sent a letter Tursday to the Security Council president "strongly condemning the Vietnamese authorities for their wanton provocations."

China and Vietnam have fought often during the last 1,000 years, as expanding Chinese empires have pushed the Vietnamese people south and Chinese emperors have regularly demanded tribute from Vietnamese kings.

In this century, however, the Chinese Communist Party provided considerable training and aid to Vietnamese Communists. During Hanoi's long war with the American-based South Vietnamese government, Chinese weapons and supplies rolled daily through the "friendship pass" on the Sino-Vietnamese border.

After the end of the Vietnam war, Peking opposed Hanoi's efforts to extend Vietnamese influence over new Communist governments in neighboring Cambodia and Laos. Massive Chinese military support for Cambodia in a bloody border war with Vietnam last year failed to prevent a blitz by Hanoi troops in December that forced the Chinese-backed government to retreat to the jungle.

Border tensions between China and Vietnam also increased last year as more than 170,000 ethnic Chinese residents of Vietnam flooded across the border into China. They said they were escaping Vietnamese persecution.

As Vietnam grew increasingly closer to China's arch-rival, the Soviet Union, and finally signed a peace and friendship treaty with Moscow, China cut off all aid and all but broke off diplomatic relations.

More and more Chinese troops, arms and aircraft were moved to the border. Yang Teh-chih, a skilled Chinese army commander who helped lead China's troops in the Korean war, was sent to take over the Kunming military region, which controls the Yunnan section of the border. The Kwangsi section is under the Canton military region, headed by Hsu Shihyu, a Politburo member and one of the most influential men in the Chinese Communist Party.

Foreign analysts have argued that any major conflict between China and Vietnam would be a severe blow to their battered economies. Vietnam has suffered a series of serious floods, one of the reasons for the massive flow of refugees out of that country. China is in the midst of an all-out effort to modernize its industry, weakened by years of internal political strife.

Chinese leaders have made contradictory statements in recent days about the border situation. Vice Premier Teng Hsiao-ping said during his visit to the United States two weeks ago that China would take no rash actions, but told Japanese officials later that Vietnam had to be taught a lesson.

The Japanese news agency Kyodo, reporting from Peking, said the Chinese leadership made the final decision on Wednesday to go ahead with an attack on Vietnam.