Seven suspected rebels have been arrested in Thailand's violence-wracked south, authorities said Saturday, as a manhunt sweeps through remote villages for gunmen who killed 15 in an ambush considered the deadliest attack in the yearslong insurgency.

Thailand's three southernmost provinces have been in the grip of a simmering 15-year conflict that has killed more than 7,000 people, the majority civilians, as Malay-Muslim militants fight for more autonomy from the Thai state.

The fighting is characterised by tit-for-tat attacks that usually target symbols of the mostly Buddhist Thai state and its security forces.

But Tuesday night's ambush in Yala province was on two checkpoints manned by civilian defence volunteers -- villagers trained and armed by the Thai state -- which left 15 of both Buddhist and Muslim faith dead.

A suspect was swiftly arrested the following day and raids carried out late Friday night in Yala and Pattani provinces netted six more, said southern army spokesman Pramote Prom-in.

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.

"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.

Armed civil defence volunteers and police have been searching vehicles at checkpoints in the southern province of Narathiwat
Armed civil defence volunteers and police have been searching vehicles at checkpoints in the southern province of Narathiwat AFP / Madaree TOHLALA

No group has stepped forward to claim responsibility in this attack, as is the case with most incidents in the so-called "deep south".

But a shadowy patchwork of rebel groups have long warred with Thai security forces, accused of heavy-handed tactics by Muslim communities who feel targeted in their homes.

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

All suspects have been moved to the notorious Inkayuth military camp in Pattani province, said Pramote -- 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".

Anger had snowballed over the case, with an umbrella group representing some of the shadowy rebel groups saying they suspected "foul play" in Abdulloh's case and calling for an international probe.