SIEM REAP, CAMBODIA -- Lying on a wooden-plank hospital bed, his left leg gone and his right leg knit together with steel needles, Liv Sen embodies the grim mood that has overtaken this traditionally fertile area of western Cambodia.

Land mines planted in roads and rice fields are crippling farmers and making it dangerous to bring in harvests. Communist Khmer Rouge guerrillas, pursuing a war-of-attrition strategy, are stepping up attacks on Cambodian government forces.

And thousands of peasants, driven by a combination of drought, floods and guerrilla attacks, are abandoning their fields to seek refuge further inside Cambodia or across the border in neighboring Thailand.

Now, despite a U.N.-sponsored peace process, the Cambodian government and three guerrilla groups arrayed against it are launching a new round of dry-season fighting that observers fear could be among the bloodiest in the 12-year-old war.

Liv Sen, a 42-year-old militiaman loyal to the Vietnamese-installed government in Phnom Penh, was injured in October while on mine-clearing duty in his village of Kvaw, 19 miles north of this provincial capital. Since then, he said, the hamlet of about 500 inhabitants was overrun by about 200 Khmer Rouge guerrillas.

"My relatives tell me the Khmer Rouge infiltrated everywhere," Liv Sen said. He said the guerrillas "told the people that they must eat their chickens and cows immediately so the Vietnamese puppets wouldn't get them. But then the Khmer Rouge took the chickens and cows for themselves."

The Khmer Rouge, the strongest of the three guerrilla organizations, is blamed for more than 1 million deaths during 3 1/2 years of rule until invading Vietnamese forces ousted them in January 1979.

Hanoi announced the withdrawal of the last of its troops in September 1989, but diplomats in Phnom Penh estimate that between 4,000 and 9,000 Vietnamese soldiers, plus hundreds and possibly thousands of government advisers, remain in the country.

The Cambodian government, the Khmer Rouge and two non-Communist guerrilla groups met in Paris last month to discuss a United Nations peace plan, which calls for a "U.N. transitional authority" to administer the country and supervise national elections.

No agreement was reached on any key issues, including a cease-fire or on disarming and demobilizing the four factions' armed forces during the plan's transition period. The Phnom Penh government, after earlier accepting the U.N. plan, has since objected to provisions for disarming its troops and dismantling much of its administration before elections.

The plan, issued in August by the five permanent members of the U.N. Security Council (the United States, Britain, France, the Soviet Union and China), calls for the United Nations to assume unprecedented military and administrative powers in Cambodia, taking over the ministries of defense, foreign affairs, finance, public security and information.

Phnom Penh recently called the plan a devious means for restoring the Khmer Rouge to power. "This is in fact their most perfidious scheme to topple the state of Cambodia through pacification," government radio said.

Cambodian President Heng Samrin later charged that the guerrillas were asking his government to "commit suicide" in the negotiations. However, diplomats say peace efforts are continuing and that Indonesia's foreign minister and a senior French official plan to visit Hanoi later this month to try to move the process forward.

Even while talking peace in Paris, both sides were stockpiling weapons in northern and western Cambodia for their latest dry-season offensives. Phnom Penh announced attacks against the Khmer Rouge in western Cambodia's Pailin gem-mining area earlier this month.

And the Khmer Rouge warned residents of Battambang, the country's second largest city, to evacuate or risk getting caught in intensified combat.

In part because of the fighting, as many as 30,000 Cambodians fled to refugee camps in Thailand in 1990, while about 150,000 others were displaced inside the country, relief officials reported. The U.N. World Food Program estimated recently that Cambodia's 8 million people would suffer a food deficit of up to 100,000 tons of rice this year, double last year's shortfall.

"The cost of the war is a big account -- too much," said Leng Vy, the governor of Siem Reap province. "We have lost at least 5 percent of our land for cultivation, the land near the jungle. People are afraid to go there because the Khmer Rouge could attack them."

He said that even in Siem Reap, better off than less fertile provinces, rice production dropped by 20 percent in 1990 compared with the year before.