EVEN after signing a peace accord this week, Cambodia's warring politicians and other concerned countries know there can be no easy peace, with or without the Khmer Rouge.Amid widespread anxiety over the future role of the fanatic Marxist guerrillas, Cambodia's four bickering factions and 18 interested countries signed a pact in Paris Wednesday, ending two decades of civil war. In a separate and expected move the United States said it would begin talks to restore ties with Vietnam within a month, but cautioned that the pace of negotiations would be dictated by Hanoi's cooperation on Americans missing in the Vietnam war. The ambiguous Cambodian peace blueprint, which thrusts the United Nations into an unprecedented administrative and peacekeeping role has spurred hope along with deep worries about a looming confrontation with the secretive Khmer Rouge. During the last two weeks before the signing, Cambodia's turbulent border with Thailand, where 350,000 refugees are sheltered, has been jolted by fresh fighting between the resistence and the Phnom Penh government. Further shocks came when the Khmer Rouge reportedly began to forceably resettle refugees as a power-base inside the country. Despite the guerrillas' pledge to participate in a rejuvenated political process, the Khmer Rouge's return to Phnom Penh, the Cambodian capital next month as part of the interim Supreme National Council (SNC) has raised concerns of their dreaded past. The Khmer Rouge are blamed for the deaths of more than 1 million people during a brutal agrarian reconstruction of Cambodia in the 1970s. The guerrillas were ousted by Vietnam in an invasion of the country in late 1978, but became the strongest of the three resistence factions opposing a Vietnamese-installed regime in Phnom Penh. Chinese assistance and Western aid to the rebel coalition further strengthened the Khmer Rouge. Under the peace plan, the newly formed UN Transitional Authority in Cambodia (UNTAC) will mobilize a peacekeeping force to monitor Cambodia's cease-fire and disarmament and will administer the country in the runup to a national election expected in late 1992 or early 1993. Prince Norodom Sihanouk, Cambodia's former monarch, heads the interim SNC, which will assist the UN as the country's sovereign body. The resistence members of the SNC, including the Khmer Rouge, return to Phnom Penh in mid-November. To the dismay of some Cambodians and Western officials, the new settlement is a compromise: It does not cite the Khmer Rouge for its excesses during the 1970s. However, Prince Sihanouk and diplomats from Asian and some Western countries insisted that the guerrillas had to be part of the political process to counter any military threat. "An accord without them would have meant a rapid resumption of fighting," French Foreign Minister Roland Dumas said this week. France was one of the sponsors of the Paris International Conference on Cambodia. Shelling still rumbles around the UN-sponsored refugee camps just inside Thailand's border, however, and Western aid workers say many Cambodians remain jittery about returning home to areas devastated by fighting and carpeted with land mines. "There is no safety net in this agreement to protect Cambodians against violations," says Raoul Jennar, a spokesman for several international aid organizations. Mr. Jennar has repeatedly warned against a return to power of the Khmer Rouge. In unusually vocal warnings in recent weeks, UN officials, who plan to begin voluntary repatriations early next year, cautioned the Khmer Rouge against forced movements of refugees. That followed the removal of 16 civilian administrators at one Khmer Rouge-controlled camp after the guerrilla officials had backed the UN plan for voluntary resettlement. The accord also raises questions about how a UN peacekeeping force will disarm 70 percent of all forces, including Khmer Rouge soldiers entrenched in the mountains in southwestern Cambodia. "I don't know whether UNTAC will have enough ability or officials to supervise the forest regions and mountains," hard-line Chea Sim told reporters in Phnom Penh this week.