A bomb explosion Thursday killed three Thai policemen returning to base after escorting teachers to school in the conflict-ridden province of Pattani, dpa reported.
The bomb, planted on a public road in Nongjik district of Pattani, 700 kilometres south of Bangkok, was detonated by a mobile phone shortly after 8 am (0100 GMT) as the pickup truck carrying police passed over it, said Lieutenant General Prasit Kirisri, commander of the joint army/police task force in the deep South.
Three policemen died instantly in the blast and fourth was listed in critical condition from his injuries.
"This attack was an effort by the militants to counter the progress we have made in winning people in this district over to the government side," Prasit said.
It was the latest atrocity in the five-year-old conflict in Thailand's deep South that has cost the state an estimated 109 billion baht (3.1 billion dollars) and resulted in the loss of almost 3,300 lives.
Deep South Watch, an independent research group that monitors the conflict, has put the southern death toll since January 2004 at 3,287 lives, of whom 1,788 were Thai Muslims and 1,348 Thai Buddhists, with another 5,405 people wounded.
Of the 300,000 Thai Buddhists who used to inhabit the region, some 70,000 have left since 2004.
Violence in Thailand's three southernmost, Muslim-majority provinces of Narathiwat, Pattani and Yala escalated after separatists raided an army depot in January 2004, killing four soldiers and making off with 300 weapons.
The incident sparked a series of brutal government crackdowns on the region's long-simmering separatist movement, which turned much of the 2-million-strong population, 80 per cent of whom are Muslim, against the central government.
Although the region, which centuries ago was the independent Islamic sultanate of Pattani, was conquered by Bangkok about 200 years ago, it has never wholly submitted to Thai rule.
Analysts say the region's Muslim population, the majority of whom speak a Malay dialect and follow Malay customs, feel alienated from the predominantly Buddhist Thai state.