WARRING Cambodian leaders have hurdled a major obstacle to ending the country's 12-year civil war, compromising on a complex and costly plan to disarm.The Cambodians, meeting in this Thai resort town, also agreed to give Prince Norodom Sihanouk, Cambodia's hereditary monarch, the final say in implementing a peace accord. They also reportedly made progress on resolving differences over a controversial proposal for the United Nations to temporarily administer the country's transition to peace. Announcing that the four Cambodian factions had agreed to demobilize their military, Prince Sihanouk urged a reluctant United States to sanction the compromise. Under a plan that would thrust the UN into an unprecedented and difficult peace-keeping role, the three Cambodian guerrilla factions and the Phnom Penh regime would demobilize 70 percent of their soldiers, weapons, and equipment, and place the remaining 30 percent under UN supervision. The reductions would cut the Cambodian armies from 150,000 to 50,000 troops, Western observers estimate, although the Phnom Penh forces would still outnumber the resistance. The Phnom Penh government, led by Prime Minister Hun Sen and backed by Vietnam, had opposed disarming, fearing the return to power of the Khmer Rouge, radical Marxists who dominate the three-party resistance. Under pressure from Vietnam, however, Hun Sen suggested a 40 percent reduction earlier this week. In return, the Cambodian leader demanded concessions on the proposed dismantling of the Phnom Penh administration. Washington, one of five permanent members on the UN Security Council which has shepherded the peace process, insists the Cambodians adhere to the UN plan, including full demobilization by the time of elections. Richard Solomon, a US assistant secretary of state, says a comprehensive peace must precede a UN presence in Cambodia. The US has feared that the rival Phnom Penh Communists and Khmer Rouge could dominate a future Cambodia under a so-called "red solution" spearheaded by their patrons Vietnam and C hina. Those fears, however, were receding after the agreement Aug. 27, Western diplomats said. DIPLOMATS from the Security Council countries, including the US, China, the Soviet Union, France, and Britain, are scheduled to meet with the Cambodians Aug. 29 and 30. "If the Americans say that they don't appreciate our work, we cannot send [the UN]," Sihanouk said, nevertheless predicting an eventual UN go-ahead. "If the Americans say it is unsatisfactory, there will be an impasse and no [UN]." Playing down differences, a US official called the plan "ambiguous," saying Sihanouk "dodged almost everything." Western supporters of the compromise admit the plan is fuzzy and faces extreme difficulties. Demobilizing Cambodia's well-armed, jungle-based guerrilla armies of unknown size will be tough, even for a massive UN force, political observers say. Sihanouk says his army includes "a few thousand" troops. Kor Boon Heng, a Khmer Rouge official, numbers his group at 30,000 to 40,000 troops and supports the disarmament plan to "ensure a neutral political process during elections." China, seeking to bury differences with rival Vietnam, has pressured the Khmer Rouge in recent months into crucial compromises. In addition to the regular Army, Phnom Penh also has large forces of security police, established during Vietnam's 10-year occupation of Cambodia, and village militias set up since the pull-out of most Vietnamese troops in 1989. When questioned about future control on the police and militias, Sihanouk floundered and then said the forces "will be kept intact but under the permanent control of [the UN]. They will not be free to operate anywhere." The UN presence, estimated to cost $2 billion, would be difficult to finance at a time of strained resources, observers say. Sihanouk says Cambodia would look to Japan for much of the funding.