China has begun moving troops and military equipment toward the Vietnamese border in a manner reminiscent of the buildup that preceded their 30-day war last February and March, according to Western intelligence sources.

Sino-Vietnamese tensions have persisted in the wake of their border conflict, but whether China intends to renew the fighting is unclear. Diplomatic sources said the Chinese were going about their buildup in a leisurely fashion, and that it probably would continue through September and October.

The most likely time for an attack, if one comes this year, would be late November or early December, when the rainy season ends and Vietnamese forces are expected to mount an offensive to try to wipe out remnants of the fallen Pol Pot government in Cambodia.

China backed Pol Pot, and Chinese anger at Hanoi's invasion of Cambodia and the toppling of Pol Pot in January contributed to China's decision to strike at Vietnam last Feb. 17.

British military sources said that elements of the Chinese 42nd Division, normally based in Guangdong Province just across the border from Hong Kong, have left their usual positions.

"Cambodia is unfinished business for the Chinese and they aren't going to rest until they finish it," a U.S. diplomat said.

One of China's objectives in its war with Vietnam was to force Hanoi to withdraw its forces from Cambodia. But China withdrew its troops from Vietnam without achieving that goal.

Vice President Mondale, who arrived in Hong Kong today after a week-long visit to China and talks with Chinese leaders, told a meeting of U.S. ambassadors to Thailand, Malaysia, the Philippines, Singapore and Indonesia, that China has reserved the right to teach Vietnam a "second lesson."