By the BBC's Simon Ingram In Bangkok

A thriving trade in illegal drugs is once again fuelling tensions between Thailand and Burma.

Recent reports spoke of sporadic exchanges of artillery fire between Thai forces and an ethnic militia said to be responsible for the mass production of methamphetamines - known as 'ice' in the West.

Thaksin Shinawatra wants Burma to act

The remote, forested hills that divide Burma and Thailand have long served as a conduit for traffickers in heroin and other illegal drugs.

It is an area controlled by the United Wa State Army (UWSA), a powerful militia group that has hitherto enjoyed close ties to Rangoon.

Until recently, Burma's military rulers turned a blind eye to the UWSA's well-documented trafficking activities.

But the junta has come under intense pressure from Bangkok to take action against its ally, and halt the flow of drugs into Thailand.

'Crazy drug'

The Thai Government's purpose became clear when prime minister Thaksin Shinawatra took office early last year.

Urine tests are performed on Thai students

He promptly declared war on a drugs problem many Thais feared was threatening to overwhelm the nation.

Thailand's biggest worry is a form of methamphetamine known as 'ya ba' - literally, 'the crazy drug'.

Cheap and easy to produce, ya ba was originally popular among truck drivers and sex workers.

But in recent years, the drug has taken a powerful hold on Thai youngsters right across the social spectrum.

Ya ba has become so pervasive that the interior ministry recently ordered schools to begin random urine tests on pupils to check for signs of drug abuse.

Abundant supply

Recent statistics issued by Thailand's Office of Narcotics Control Board put the number of amphetamine and heroin addicts at 1.2 million.

The drugs are made in factories in Burma

Most of the ya ba is allegedly made in small factories scattered across border areas controlled by the UWSA.

Thai officials say the plants - many of them controlled by drug barons like Wei Xhieu Kang - currently produce as many as 700 million ya ba pills a year, most of which will end up on sale in Thailand.

Other ya ba factories operate in Laos and Cambodia.

Spectacular seizures by the Thai authorities have become almost routine, but have done little to stem the flow of drugs into the country.

In January, Bangkok police discovered a pick-up truck containing two million ya ba tablets on the city outskirts.

An even bigger seizure was made by the army on the border in April, following a clash with armed traffickers.

Image problem

The involvement of corrupt Thai officials and military officers has made the trade even harder to combat.

Until recently, the Burmese authorities either denied that the Wa's narcotics production posed a serious problem, or else claimed they could do nothing about it.

But by early this year, uncomfortably aware of the damage the issue was doing to Rangoon's attempts to rehabilitate its international image, attitudes among ministers were showing a change.

In January, Burma joined Thailand, China and three other nations bordering the infamous Golden Triangle drug-producing region in pledging a joint effort to tackle the narcotics traffickers.

The same month, Foreign Minister Win Aung said that efforts to persuade the Wa to renounce drug production were bearing fruit.

"The Wa leadership is becoming more and more aware of its responsibility to stop (drug trafficking)," the minister told the Nation newspaper.

"They have assured us - and we believe them - that they will stop."

Action required

Soon, Thai officials were noting an improved record of cooperation from the Burmese.

Methamphetamine abuse is a serious problem

The United Nations Drugs Control Programme (UNDCP) even described Burma as one of the most committed states in the battle against drugs.

But many remain sceptical as to whether Wa leaders can be trusted to fulfil their promise of making their territory drug-free by 2005.

By ratcheting up military pressure on the UWSA, Thailand appears to be sending a firm reminder to Rangoon that promises not backed up by action are not enough.