Just days after southern Thailand's first bomb attack against a tourist target, the theft of a large quantity of ammonium nitrate and explosives by Islamic militants has raised the prospect of a major terrorist strike against civilian or tourist infrastructure, regional security sources say.

The theft on the evening of 30 March involved a raid by fewer than 10 masked insurgents on a quarry in Muang (central) district of Yala province. Clad in black fatigues and armed with assault rifles, the insurgents bound, gagged and blindfolded two elderly, unarmed watchmen and made off with between 1.4 and 1.65 tonnes of ammonium nitrate in 66 sacks; 56 sticks of dynamite; and 176 detonators - potentially enough to level a large building. In April, it was reported that Thai authorities had recovered roughly half the stolen ammonium nitrate, which had been hidden in a deserted mine nearby, and arrested seven people for questioning. However, that still leaves a large amount of explosive in the hands of the militants.

The raid bore many of the hallmarks of the 4 January attack on an army base in neighbouring Narathiwat province in which attackers seized a total of 413 firearms. That raid triggered the imposition of martial law across the mainly Muslim-Malay border provinces of Pattani, Yala and Narathiwat, and marked the beginning of a sustained campaign of assassinations and arson attacks by Muslim insurgents that has killed some 60 people this year. Like the arms seizure, the raid on the quarry was executed with military precision: the attackers withdrew with the explosives loaded onto two pick-up trucks within 20 minutes, with several roadblocks laid to cover their retreat - a tactic used in the 4 January raid.

The operation was also the product of good intelligence and careful planning. In theory, explosives used in quarries in the region - there are 20 in the three provinces - are held in bases of either the Royal Thai Army (RTA) or Border Patrol Police (BPP) and only released to quarries immediately before use. In this case, the raiders were aware either of a short window of opportunity available between the release of the explosives and their use for rock blasting, or that regulations were being breached and a large supply was being held at the quarry without effective security.