Bangkok, Thailand — Beyond Chiang Rai in the top northwestern corner of Thailand, the green-shrouded mountains are alive with the sound of loudspeakers blaring from circling helicopters.
The area, close to borders with both Burma and Laos, has been the scene of renewed fighting between Thai forces and the private army of opium warlord Khun Sa. Thai helicopter loudspeakers are calling on Khun Sa's men to surrender.
The new drive against Khun Sa has occurred on both sides of the Thai-Burma border. It is the first action of its kind undertaken in coordination by the the Thai and Burmese governments. This followup to a January attack was also a sign of the Thai government's continuing commitment to keeping up the pressure on a drug kingpin in the ''Golden Triangle.'' This mountainous and difficult to control area, framed by the borders of Thailand, Burma, and Laos, is known as Southeast Asia's major supplier of opium.
Poppies, widely cultivated by minority tribes in the area, produce the opium which is eventually refined into heroin for use by addicts in Asia, Europe, and the United States.
In March, when Thai ministers visited Rangoon, they were informed of Burma's planned ''Operation Thunderbolt'' against Khun Sa, timed for early May. The Thai action was launched to coincide with it.
Satellite pictures had also revealed to the Thai government a new stronghold built by Khun Sa's men at Ban Laolu, a little north of his old headquarters at Ban Hin Taek. Thai forces overran this earlier stronghold in January.
When Thai units moved into Ban Laolu a few days ago, they were amazed to find a village of 100 buildings replete with sophisticated equipment. One senior Thai official in Bangkok said Khun Sa had been invited to return by ''influential people.'' He refused to elaborate, but was probably referring to Khun Sa's friends in high places in Thailand.
One of the best-informed Western narcotics operatives in Southeast Asia reports: ''Thai attacks on Khun Sa disrupted his operations so that heroin is harder to find in parts of Thailand, particularly in the north. Against that we know new refineries have opened in the south. So, you plug the hole in one place , only to find others opening elsewhere.''
Khun Sa is believed to be back on Thai territory, almost certainly at Lao Lo Chai a town which spills into Burma. Control by the central government there is uncertain. The warlord's most important heroin refineries are close by.
A new factor perplexing anti-narcotics agencies is the apparently growing involvement of communist insurgents in the drug trade. Evidence has been mounting for some time that the Burmese Communist Party (BCP), and to a lesser extent the Communist Party of Thailand (CPT), are financing their insurgencies with drug profits.
Processing, as well as production, of opium is increasingly being taken over by the BCP, according to a report to the US Senate Foreign Relations Committee by the US State Department's assistant secretary for narcotic matters, Dominic DiCarlo.
One reason may be that China has reduced its aid to the Burmese communist insurgents in a bid to improve relations with the central government in Rangoon. This may have put pressure on the insurgents to turn to drugs for revenue.
The BCP now appears to seek a share in the lucrative processing and distribution of narcotics dominated by Khun Sa. The clash of interests is already causing disputes between Khun Sa and the BCP, according to agents who do not rule out the possibility of open conflict between the two.
The BCP has 12,000 soldiers backed up by a militia of 30,000. It controls a plateau which is the largest center for opium cultivation in Southeast Asia.
Western and Thai narcotics officials claim also to have near conclusive evidence that the communist government of Laos is also engaged in drug production and trading to boost its impoverished economy.
Similarly, the hardpressed Communist Party of Thailand (CPT), suffering from military setbacks and the loss of Chinese aid, is believed to be making money from opium and heroin in the extreme north and south of Thailand.