At least 40 people were wounded on Tuesday after two bombs detonated outside a shop in the Pattani province of Thailand's violence-plagued south, police said.

The first explosive was packed inside a motorcycle parked in the car park, spreading panic among shoppers.

"The second blast was a huge car bomb, which occurred five minutes later and injured at least 40 people, while one is in a serious condition," said Captain Preecha Prachumchai of Pattani Police.

Pictures posted by rescue workers showed a thick plume of black smoke over the supermarket car park and a damaged store entrance.

Police were on the scene and appeared to be encouraging people to move back when the second blast struck.

"I heard a very loud explosion. Minutes after that, I heard the sirens of rescue cars and ambulances," said a resident who lives near the supermarket, on condition of anonymity.

"I feel bad about it … it happened at a place where people go to buy things."

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The car bomb could be seen in a video clip that went viral on social media, although the car in question was not visible.

Pattani Police suspect hardline rebels of carrying out the attack, but no culprit has yet been identified.

"The bomber was believed to have fled the scene before the explosion. Investigations are under way," said Rewat Srichantub, Pattani's Deputy Police Chief.

Increase in violence

Thailand is mostly Buddhist, but parts of the south are majority Muslim. The region has been plagued by a long-running separatist insurgency as ethnic Malay rebels battle Thai troops for more autonomy from the Buddhist-majority rule.

Attacks in Thailand's deep south have intensified over the past few months. Six army rangers were killed in an attack last month.

According to Deep South Watch, an independent monitoring group, daily shootings and bomb attacks have claimed more than 6,500 lives since 2004, with both sides accused of rights abuses.

The biggest faction spearheading the insurgency is the Barsian Revolsui Nasional (BRN).

The government recently rejected a conditional offer for peace talks from BRN, which demanded international observers or mediation and protections for the Malay-Muslim culture.