Gunmen opened fire on security forces and civilian volunteers at a checkpoint in Thailand's restive south on Tuesday night, killing at least 15 people in what is believed to be the deadliest single attack in the region in years.

More than 7,000 people have been killed since a separatist rebellion started in southern Thailand in 2004, according to Deep South Watch, which monitors violence there. The region is predominantly Muslim and was annexed from Malaysia by Buddhist-majority Thailand more than 100 years ago.

"This is likely the work of the insurgents," Col. Pramote Prom-in, a military regional security spokesman, told Reuters.

The floor of the checkpoint was soaked in blood and the walls covered in bullet holes, as forensic police combed through the grisly debris Wednesday morning. Most of the dead were village defense force volunteers in Yala Province, as reporter Michael Sullivan told NPR from Chiang Rai, Thailand. At least five other people were wounded

Security officials say the unidentified attackers tried to make it difficult for victims to escape and for rescue workers and security backup to get to the scene. "The insurgents scattered road spikes, set fire to tyres, felled a tree and bombed a power pole to obstruct pursuit," the Bangkok Post reported.

About 10 gunmen sneaked through a rubber plantation in Yala Province to approach the checkpoint, according to the newspaper. During the attack, they stole "an assault rifle, two shotguns and five pistols from the checkpoint victims."

Army spokesman Col. Kiattisak Neewong told The Associated Press that officials recovered bloody clothes at the scene, a sign that some of the attackers may have been wounded. Four of the people killed were women and one was a doctor, according to the spokesman.

There were also reports of a second checkpoint attack on the same night, according to thePost, though nobody was reported to have been injured there.

Thai Prime Minister Prayuth Chan-ocha has vowed to better protect these volunteer forces at checkpoints, according to the AP, saying Tuesday night's attack could be an indication that rebels are planning to target them more frequently.

"Normally the insurgents don't hit these village volunteers because they are considered civilians, unless they [cross] the line and become part of state apparatus," Don Pathan, an expert on southern Thailand, told Reuters.

The rebels are seeking greater autonomy or even independence in southern Thailand. "The southern provinces are among the country's poorest and least developed," as Sullivan reported over a decade ago, soon after the violence started. "Many Muslims in the south say that's no accident. They accuse Bangkok of systematic neglect and discrimination."

It's not clear which group carried out this attack, though the attack is similar in style to others carried out by the separatist group Barisan Revolusi Nasional, the BBC reported.

Since 2013, the number of violent incidents per month in the south has slowly ratcheted downward, as has the number of dead or injured, according to data from Deep South Watch.

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