THE United Nations Security Council this week approved the start of a major peacekeeping effort in war-torn Cambodia, expected to become the largest and most expensive UN operation ever.After the Cambodian parties sign a comprehensive political agreement in Paris next week, the UN will move in to demobilize and disarm the guerrilla groups, and organize elections for an assembly that will draft a constitution. To ensure that the process is free and fair, the UN will take over five of Cambodia's key ministries: foreign affairs, defense, finance, public security, and information. This is the first time the UN has played such a role. UN peacekeepers will also help clear hundreds of thousands of mines planted during the past two decades of war, which continue to cause civilian casualties. The Council voted unanimously Wednesday to dispatch an advance party of 200 military and civilian observers to oversee a cease-fire between three guerrilla factions and the Vietnam-backed government in Phnom Penh. The UN advance mission will provide security for the return of Prince Norodom Sihanouk, the former hereditary leader ousted in the early 1970s. The prince is now the elected president of the Supreme National Council, which involves all Cambodian factions including: * The anticommunists who overthrew Sihanouk and ruled with US support until 1975. * The Khmer Rouge, who took over in 1975, emptied the cities, and sent the urban population to cultivate rice fields in a frenzy of self-reliance that cost hundreds of thousands of lives. * The rival Cambodian Marxist faction installed by Vietnam in Phnom Penh in 1979. The Supreme National Council took Cambodia's formerly contested seat at the UN last month. The SNC and the UN will jointly administer Cambodia during the transition period. As part of the agreement, Cambodia is to undertake "effective measures to ensure that the policies and practices of the past shall never be allowed to return." In a major policy shift, the UN has announced that its High Commissioner for Refugees will take over operations along the Thai-Cambodian border as of Nov. 1. Thailand has previously refused to allow the 350,000 Cambodians who sought refuge on its territory to be classified as refugees, in order to prevent their permanent settlement in Thailand and to obstruct resettlement to third countries. Now, UN officials say, the Cambodians will be allowed to move out of limbo, but in one direction only - back to Cambodia.