AFTER last week's attack on a Khmer Rouge leader, the United Nations is under pressure to protect the widely reviled guerrillas and salvage its shaky Cambodian peace.The four rival Cambodian factions comprising an interim reconciliation council return today to Thailand's beach town of Pattaya to grapple with the security threat to Khmer Rouge leaders. Contending that the peace plan risks failure without including the Khmer Rouge, analysts say the UN could be cornered into guarding the notorious guerrillas, who are blamed with the deaths of at least 1 million Cambodians in the 1970s. Under an October accord, the UN will temporarily administer Cambodia during an 18-month transition, monitor a cease-fire, demobilize most of the armies, and oversee elections. After the mob attack on the Khmer Rouge's Khieu Samphan, that responsibility may be expanded. "The UN will have to get into the act," says a Western diplomat in Bangkok who follows Cambodian developments. "The international community will have to do the unthinkable if [it] wants this agreement to work." The mob assault has left Cambodia, a nation battered by war for two decades, in a new limbo. Demanding guarantees of safety, the Khmer Rouge nevertheless renewed its commitment to the peace plan. Western and Asian political observers say that, although some skirmishing continues between Phnom Penh government troops and the Khmer Rouge, the guerrillas are believed to be pursuing a longer-term plan to capitalize politically on mounting official corruption and widening gaps between rich and poor. The group's key backer, China, warned that the accord is in trouble. But, like the guerrillas, Beijing stopped short of condemning the Phnom Penh government, supported for years by Vietnam. This year's rapprochement between China and Vietnam, which fought a brief border war following Hanoi's invasion of Cambodia in late 1978, was the key to initialing an agreement after years of tortuous diplomacy. "China isn't ready to sacrifice its peacemaker role and international goodwill," says a Western analyst. "The Khmer Rouge seems ready to swallow this humiliation for longer-term interests." While Phnom Penh Prime Minister Hun Sen has been blamed for lax security leading up to the mob attack, he has also played to Western sympathies by further isolating the Khmer Rouge. In arriving for the Pattaya meeting yesterday, Hun Sen insisted there is no way to guarantee Khmer Rouge safety, given Cambodian anger at the human holocaust the country endured under the group's brutal rule. Most tarnished by the incident is Prince Norodom Sihanouk. The former monarch, whose deft diplomacy led to the final peace agreement, put his own reputation on the line by convincing the Khmer Rouge to sign the accord. But Prince Sihanouk has been undermined by a hasty alliance between his son, Prince Norodom Ranariddh, and the Phnom Penh government, as well as the recent violence against the Khmer Rouge. "Hun Sen is very, very smart. He's a lot smarter than Sihanouk," says a Cambodian analyst in Phnom Penh. "Hun Sen is now the main game in town."