Five days of fierce fighting between Thai forces and troops of the Golden Triangle's biggest narcotics warlord have caused scores of casualties on both sides and severely disrupted the area's drug-trafficking, Thai police and Western diplomats said today.

The fighting has pitted about 1,000 Thai Border Patrol Police backed by two companies of Army troops against several hundred fighters of the Shan United Army, a Burmese rebel group led by narcotics kingpin Chang Chi-fu.

The rebel leader, who says he seeks independence from Burma for 80,000 Shan tribesmen, so far has eluded capture.

The battle marked the first time that Thai forces have launched a serious and concerted attack against any of the bands of traffickers who have operated with relative impunity for years in the area overlapping Thailand, Burma and Laos.

The offensive was welcomed by U.S. drug enforcement officials here, who have long been encouraging Thailand to crack down on Chang's group.

In addition to the attack by Thai ground forces, the offensive involved bombing and strafing by U.S.-supplied OV10 Bronco counterinsurgency planes and helicopter gunships, officials said.

Thai authorities said their forces managed to wrest control of Chang Chi-fu's stronghold in the village of Ban Hin Taek about 490 miles north of Bangkok. The Thai forces today were sweeping the jungle north of the village, which lies a few miles from the Burmese frontier in rugged country near the point where the borders of the three Golden Triangle countries converge.

The Thai chief of police, Gen. Suraphol Chulaphram, told a news conference today that the bodies of 32 Shan United Army rebels had been recovered and that about 50 others were believed killed in bombing raids. He said 16 Thai Border Patrol Police were killed and 45 seriously wounded.

Police Maj. Gen. Pow Sarasin, director of Thailand's Office of the Narcotics Control Board, said the offensive would cut the border area's heroin production in half, but he declined to give any figures. U.S. drug enforcement officials have estimated that last year's bumper opium crop of about 600 metric tons from the three countries combined could yield about 30 tons of heroin.

However, the actual production at heavily guarded heroin refineries in the jungles just across the Burmese border has been estimated lately at 10 to 12 tons, partly because of a Thai ban on chemicals used to manufacture the drug.