China has warned Vietnam to halt "armed provocations" along their common border or suffer "grave consequences," diplomatic sources here said.

The note was seen here as a response to a reported buildup of Vietnamese troops near the Thai-Cambodian border where guerrilla groups fighting the Vietnamese occupiers of Cambodia have bases.

The Chinese statement, the strongest of a series issued to Hanoi, was conveyed to the Vietnamese Embassy here earlier this week and accused the Vietnamese forces of carrying out more than 70 border attacks in March, killing or wounding 14 Chinese. China and Vietnam fought a border war in 1979 shortly after Hanoi sent its troops into Cambodia.

The Soviet-backed Vietnamese forces "could annihilate these strongholds if they chose to do so," said an Asian diplomat. "So the Chinese want to send a signal to the Vietnamese not to go too far."

The warning, said the Asian diplomat, "is clearly related to the situation on the Thai-Cambodian border. If there is a further intensification of the Vietnamese attack there, the Chinese may resort to some kind of military retaliation on the Sino-Vietnamese border."

China is believed to stand ready to undertake military action on the Sino-Vietnamese border in the event Thai security is seriously threatened, diplomatic sources here said. In Cambodia, China acts as the main arms supplier to the three guerrilla groups fighting the Vietnamese in Cambodia, especially to the Communist Khmer Rouge faction headed by former Cambodian government leader Pol Pot. The United States supplies medicine, food and other forms of humanitarian aid to the two noncommunist guerrilla groups, led by Prince Norodom Sihanouk and Son Sann.

Peking's blunt protest note to Hanoi said, "If the Vietnamese authorities act willfully and let the trend on the Sino-Vietnamese border continue, they must be held responsible for the grave consequences." This followed a statement by Gen. Yang Dezhi, China's military chief of staff, on a visit to Thailand in February in which he reportedly pledged Peking's full support for Thailand in the event of a Vietnamese invasion.