Attack by unknown assailants on checkpoint in Muslim-majority province kills 15, wounds five more.

At least 15 people have been killed in an attack on a security checkpoint in southern Thailand, including a police officer and many village defence volunteers, according to an army spokesman.

The incident late on Tuesday was the worst single attack in years in a region where a separatist campaign has killed thousands.

The attackers, in the province of Yala, also used explosives and scattered nails on roads to delay pursuers.

Colonel Pramote Prom-in, spokesman for the army, said on Wednesday that “12 were killed at the scene, two more died at the hospital and one died this morning”. Five others were wounded, he told AFP news agency, adding that the attackers took M-16 rifles and shotguns from the checkpoint.

“This is likely the work of the insurgents,” he told Reuters news agency. “This is one of the biggest attack in recent times.”

There was no immediate claim of responsibility, however, as is common with such attacks.

Prime Minister Prayut Chan-O-Cha said the perpetrators must “be brought to justice”, according to Defence Ministry spokesman Kongcheep Tantravanich.

A decade-old separatist campaign in Thailand’s largely ethnic Malay-Muslim provinces of Yala, Pattani, and Narathiwat has killed nearly 7,000 people since 2004, says 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.

The region is under martial law, heavily policed by the military and sometimes staffed with trained civilian volunteers, with residents and rights groups accusing them of heavy-handed tactics.

Some rebel groups in the south have said they are fighting to establish an independent state. Police, teachers and other government representatives are often targets of the violence.

Authorities arrested several suspects from the region in August over a series of small bombs detonated in Bangkok, the capital, although they have not directly blamed any armed group.

“Since 2004, these attacks have intensified. They’ve become much more coordinated and larger in nature,” said Al Jazeera’s Wayne Hay, reporting from Bangkok.

“There are several groups operating there, and the largest one is called Barisan Revolusi Nasional or BRN. They are believed to be behind most of the violence in the south. They say their motivations are torture, human rights abuses carried out by the military, which operates with impunity in the southern provinces.”

In August, BRN told Reuters it had held a secret preliminary meeting with the government, but any step towards a peace process appeared to wither after the deputy prime minister rejected a key demand for the release of prisoners.

Tensions also spiked rose in the region in the south over allegations that 32-year-old Abdullah Esormusor, a Muslim man, was beaten so badly during military interrogation that he fell into a coma. He later died of his wounds.

The army has said there is no proof of torture.

Mara Patani, an umbrella group representing some factions of the armed rebels, has called for international intervention after the Abdullah case – a request rejected by Thailand’s army.