Image copyright Getty Images Image caption A relative of a volunteer who was killed in the attack

Suspected separatists have killed at least 15 people and injured four in southern Thailand, in one of the country's worst attacks in years.

Police officers were among those killed at the attack on a security checkpoint in Yala province.

The attackers used explosives and then scattered nails on roads in an attempt to slow down people chasing after them.

Thousands have been killed in southern Thailand since a separatist insurgency gripped the region in 2004.

What happened in the attack?

The attack took place on Tuesday night - Yala is one of three provinces in the region that have often been hit by violence.

Separatist insurgents are believed to have targeted a security checkpoint, killing at least 15 people - including a police officer and numerous village defence volunteers.

These volunteers are usually civilians who have been trained to use weapons and are tasked with protecting the village.

Some of the injured remain critically ill, according to the BBC's Thai Service.

There is still no evidence as to who was behind the attack, but Colonel Pramote Prom-in, a regional security spokesman, told Reuters that it was "likely the work of insurgents".

"This is one of the biggest attacks in recent times," he said.

What is the separatist movement about?

The separatist rebels seek some form of independence in southern Thailand.

To understand why, you have to go back at least 100 years.

The southern region used to be a sultanate known as Patani. Most people who lived in Patani were Malay Muslims. But at the beginning of the 20th Century, Patani became part of Thailand.

"[The Malay Muslims were] different from most people in the rest of Thailand, whether culturally, ethnically or religiously," Rungrawee Chalermsripinyorat, an independent analyst based in Australia, told BBC News.

These Malay Muslims fought "against cultural assimilation, [but were met] with strong state suppression", she said.

So they're still rebelling now?

Technically, they've never stopped. But violence in southern Thailand had relatively calmed down - until 2004.

In that year, gunmen raided a military arms depot, killing the four guards and making off with about 400 assault rifles. Waves of attacks blamed on ethnic-Malay separatists would later follow.

Image copyright TUWAEDANIYA MERINGING Image caption Authorities and insurgents have clashed in southern Thailand for more than a decade

The fight has not stopped since, though there are spikes in the violence as well as periods of lulls.

Tuesday's attack is said to be the biggest in years in terms of casualties, but it's still not clear who carried it out and why.

Ms Chalermsripinyorat suggested the attack could have been carried out by Barisan Revolusi Nasional (BRN), one of three main separatist groups in the region.

"While there is no solid evidence... this style of attack and choice of target fits the pattern the BRN has previously used," she said.

But could it be a signal that violence in southern Thailand might be returning to the levels of 2004?

"I think it is unlikely to signify a return to the full-fledged military battle [of] several years ago," said Ms Chalermsripinyorat.

"It may just be a normal military operation to remind people of [the group's] existence and military might."

Why does all this matter?

More than 7,000 people in southern Thailand have died since the conflict between the insurgents and authorities resumed in 2004, said Deep South Watch, a group that monitors the violence.

"It had received little attention internationally [but] it is not an exaggeration to say this is the most deadly conflict in South East Asia," said Ms Chalermsripinyorat.

Image copyright Getty Images Image caption Yala has seen waves of attacks over the past 15 years

But it's not just Thailand that has cause for concern. She adds that Thailand's neighbours Malaysia are "afraid of the spill over effect" and that the conflict could cause "penetration by transnational jihad groups".

"Secessionist conflicts elsewhere have shown that the longer a conflict is prolonged, the more complicated the situation becomes," she said.

"It can then become a fertile ground for unwanted elements."