UNITED NATIONS (AP) _ Vietnam's ambassador to the United Nations said Tuesday there was a Chinese military buildup on the border and ''the danger of a war of aggression by China against Vietnam is approaching.''

Hoang Bich Song said the Chinese have massed 20 combat divisions on its border with Vietnam along with dozens of artillery units and hundreds of combat aircraft.

Hoang told a news conference the civilian Chinese population has been ordered evacuated from the border area.

He said that the 20 divisions were not as many as had been deployed on the border by Chinese before their 1979 invasion of Vietnam. Without giving details, Hoang said that Vietnam has also reinforced its forces along the border.

But he said Vietnam would not launch a pre-emptive strike

He said the Chinese threat was partially aimed at relieving pressure on Cambodian rebel forces who have been the target of an offensive by Vietnamese forces in Cambodia.

In recent weeks Vietnamese forces supporting the regime of Heng Samrin in Cambodia have overrun bases near the Thai border. The Vietnamese have inflicted serious losses on forces of the coalition led by Prince Norodom Sihanouk, which holds Cambodia's United Nations seat but little else in the country.

Sihanouk has reportedly asked the Chinese to help his forces by ''teaching the Vietnamese a second lesson,'' a reference to the 1979 invasion.

Hoang said the Chinese are not influenced by Sihanouk. He said their principle aim is to counter the growing strength of the three Indochinese nations, Vietnam, Laos and Cambodia who are ''an obstacle to their ambitions.''

He said there is an ''increasingly serious possibility of war between China and Vietnam.''