Cambodian government forces backed by artillery and tanks struck guerrilla positions in central and western Cambodia on Wednesday, just as a cease-fire was to begin, a guerrilla spokesman said.

Long-range artillery of the Vietnamese-backed government pounded positions of Prince Norodom Sihanouk's non-communist guerrillas in central Kompong Thom province at about 4 a.m., said a Sihanouk spokesman in Bangkok, Ek Serey-wath.Two hours later, government troops launched a ground attack against Sihanouk army positions in Srey Snam and Varin in western Siem Reap province, he said, quoting reports from the high command.

"Of course we are fighting back," Ek said.

The high command also reported fighting Wednesday in the western Malai area bordering Thailand, which is held by communist Khmer Rouge guerrillas.

Tank-backed troops also struck the third group in the guerrilla coalition, the non-communist Khmer People's National Liberation Front, seizing three positions south of the strategic Thmar Puok town in western Battambang province, Ek said.

The Vietnamese-installed government's sole interest in the truce, Ek said, "is so they will have . . . the capital, and we will leave them alone."

There was no immediate government response to the guerrilla claims.

Vietnam and the Cambodian government have said that the temporary truce should be accompanied by a halt in outside military aid.

The Soviet Union and Vietnam arm the government side, while China and other countries supply the guerrillas through Thai territory.

All combatants had said they would honor the truce that was to begin Wednesday, which would have been the first in the 12 years of war since Vietnam's invasion of Cambodia. But there is no means of monitoring the cease-fire, and both sides had expressed pessimism that it would hold.

The truce was jointly proposed last week by France, Indonesia and U.N. Secretary General Javier Perez de Cuellar - the main mediators in the Cambodian peace process.

They said fighting was to be halted until at least the end of peace talks expected this month in Jakarta, Indonesia.