Manhunt under way over security checkpoint assault in Yala province that killed 15 people, including defence volunteers.

Authorities in Thailand have arrested seven suspects as a manhunt continues for the gunmen behind the worst attack in years in the country’s restive south.

Fifteen people, including a police officer and many village defence volunteers, were killed on Tuesday night when suspected separatists stormed a security checkpoint in Yala province.

A suspect was arrested the following day and raids carried out in Yala and Pattani provinces led to the detention of six more, southern army spokesman Pramote Prom-in said on Saturday.

Authorities also found bloodied gauze in the home of a local village doctor near the crime scene, which is under “forensics” investigation to match the blood traces left from the shoot-out, he told AFP news agency.

“We suspect around 30 to 40 people were involved,” Pramote said, adding that it remains unclear which particular separatist group orchestrated this highly organised attack.

No group has claimed responsibility for the attack, as is the case with most incidents in this area.

A separatist campaign in Thailand’s largely ethnic Malay-Muslim provinces of Yala, Pattani and Narathiwat has killed nearly 7,000 people since 2004, according to Deep South Watch, a group that monitors the violence.

The population of the provinces, which belonged to an independent Malay Muslim sultanate before Thailand annexed them in 1909, is 80 percent Muslim, while the rest of the country is overwhelmingly Buddhist.

Muslim residents have long protested that they are treated like second-class citizens. Heavy-handed government efforts to fight the separatists have increased discontent while making little obvious headway to stem the violence.

The region is under martial law, with numerous checkpoints dotting remote villages and security forces given the right to detain any person without warrant.

Pramote said all suspects have been moved to the notorious Inkayuth military camp in Pattani province, the army’s biggest detention centre in the south where rights groups have documented torture.

A Muslim man who was detained there in July was left in a coma after an interrogation session. Abdulloh Esormusor died in August and an army probe found that his death could have been due to “suffocation”.