Vietnam said yesterday that it was withdrawing "a significant number" of troops from Cambodia.

Foreign Minister Nguyen Co Thach said in Ho Chi Minh City that a further reduction in the estimated 180,000 to 200,000 troops in Cambodia would be made if Thailand would stop providing aid and a haven for Cambodian groups battling for political power in Phnom Penh.

Thach spoke following the biennial meeting of the foreign ministers from the three Communist states of Indochina--Vietnam, Laos and Cambodia. His announcement coincided with a symbolic visit to Cambodian soil by Prince Norodom Sihanouk, former Cambodian head of state and the leader of the recently formed coalition of three factions opposed to Vietnam's political and military dominance in Cambodia.

Thach's announcement in Ho Chi Minh City (formerly Saigon) was coupled with a communique issued by the Indochinese foreign ministers that urged the establishment of a "peace zone" along the Thai-Cambodia border and the convening of a major international conference on Southeast Asia.

The initial reaction by analysts in Washington to the current flurry of political and diplomatic activity regarding Cambodia by opposing forces was that it seemed designed primarily to prepare the ground for the annual struggle for Cambodia's seat in the United Nations General Assembly this fall. That seat is currently held by the Communist Khmer Rouge, former allies of Vietnam who were ousted by invading Vietnamese forces in early 1979. The Khmer Rouge are now part of Sihanouk's anti-Vietnamese coalition.

The moves by the Communist allies seemed also aimed at splitting the five members of the Association of Southeast Asian Nations (ASEAN)--the Philippines, Thailand, Malaysia, Singapore and Indonesia--which are known to differ on how open they should be to a political solution of the situation in Cambodia. ASEAN has been a major force behind the organization of anti-Vietnamese Cambodians. The coalition has also had the backing of China and the United States, the two countries that led a campaign to put Vietnam in political and economic isolation because of its invasion of Cambodia.

The political activity follows Vietnam's spring offensive against opposition guerrillas operating near the Thai border. The military action reportedly inflicted severe damage on Khmer Rouge forces but did not conclusively defeat them.

Foreign Minister Thach would not say how many troops Hanoi planned to withdraw from Cambodia. He said that an unspecified number of soldiers were pulled out last September. The Associated Press, reporting from Ho Chi Minh City, quoted Vietnamese diplomats as saying that the two withdrawals would bring their forces in Cambodia down to 150,000.

In their call for a "safety zone" along the Thai-Cambodian frontier, the Indochinese foreign ministers said it could be established following the removal of Khmer Rouge and Cambodian refugee camps from the area.

The zone would be policed by Thais on their side and by the Phnom Penh government on its side, and "Vietnamese troops would not be deployed there," the statement said. The zone could be supervised by the United Nations once the world body ousted the Khmer Rouge from the General Assembly, the foreign ministers said. Thailand has previously rejected a call from the Communist states for a demilitarized zone along the border on the ground that it violated its sovereignty.

Thai officials were quoted in Bangkok as saying that the government would have no official comment on the statement until the text had been carefully studied.

The Communists' proposal for an international conference on Southeast Asia would include Vietnam, Laos, Cambodia (represented by the current Phnom Penh government), the ASEAN states, India, Burma and the five permanent members of the U.N. Security Council--China, the Soviet Union, the United States, France and Britain.

In his first visit to Cambodia in three years, Prince Sihanouk walked about 100 yards from the Thai border to Sroch Srang, a camp of guerrillas and refugees allied with former premier Son Sann, who heads a faction of the opposition coalition formed last week.

During his stay Sihanouk urged Cambodians to unite to throw out the Vietnamese "colonialists" and establish a democratic, parliamentary system in the country.

Sihanouk, who heads the Moulinka faction in the coalition, said he intends later this week to visit a camp of the Khmer Rouge, the third member of the grouping.

Son Sann, leader of the Khmer People's National Liberation Front, greeted Sihanouk and said that a merger of his group with Sihanouk's was "only a matter of time." The two have done little to hide their antipathy toward joining forces with the Khmer Rouge, whose 3 1/2 years of iron-fisted rule in Cambodia are believed to have resulted in the death of as many as 3 million people.