Sharply disagreeing with his coalition allies, Prince Norodom Sihanouk says they are unrealistic when they talk about expelling the Vietnamese from Cambodia.

But despite the bleak prospects for his country, the former Cambodian ruler said the coalition has no choice but to fight on.

In a recent interview here, Sihanouk indicated that this year's Vietnamese attacks on the border camps of the coalition's noncommunist guerrillas had done nothing to bring the coalition's three partners closer together.

Sihanouk heads a shaky U.N.-recognized coalition made up of the communist Khmer Rouge and two noncommunist groups, one loyal to himself and a larger group led by former Cambodian prime minister Son Sann. The coalition has been fighting the Vietnamese occupation for more than two years.

Sihanouk is the best known of the leaders, but in the continuing war for Cambodia he has changed sides so many times that some observers have questioned his credibility.

Nevertheless, the former ruler seemed to speak more openly than before about the distrust among the coalition partners. He made scathing remarks about the military capabilities of the noncommunist guerrilla group headed by Son Sann.

"You need Son Sann because he is an honorable man," Sihanouk said. "But you know that Son Sann and his generals are zero on the battlefield. Pol Pot is no zero."

The Khmer Rouge, under Pol Pot, ruled Cambodia until the Vietnamese invasion and are widely viewed as responsible for the deaths of hundreds of thousands of Cambodian civilians.

Pol Pot is "now the only man capable of weakening the Vietnamese . . . not winning, but bleeding," Sihanouk said. "So everybody needs Pol Pot." But the Khmer Rouge and Son Sann's group "hate each other very, very much," he said. "They distrust each other very much."

Although all three factions are in the coalition against the Vietnamese, the two noncommunist groups are seeking their own weapons from the West to sustain an independent power base.

Son Sann and Prince Norodom Ranariddh, Sihanouk's son, who were in Washington earlier this week to seek U.S. military aid for their fighters, played down the differences within the coalition.

Ranariddh said the coalition's objective was to "make the Cambodian water hotter and hotter for the Vietnamese fish, to bring them to the table for a political solution."

The House Foreign Affairs Committee authorized $5 million in military aid to the noncommunist resistance last week. Since then, administration officials, in a policy change, said they would not rule out the possibility of granting aid if it is approved by both houses of Congress.