Bangkok - An explosion Friday in a grocery store in Thailand's violence-ridden deep South killed five civilians and injured 10 policemen who had arrived to inspect the scene of a robbery.

The bomb went off at 10:40 am as police arrived to investigate a robbery at a store in Dusongyor town of Narathiwat province, about 800 kilometres south of Bangkok.

Five civilians were killed on the spot by the blast, including the shop owner's brother, while 10 policemen and two other bystanders were injured, said The Nation online news service.

Police said the bomb was assembled inside a cooking gas cylinder and detonated with a mobile phone.

The country's deep South, where more than 2,700 Thais have died violently over the past four years, has been wracked by a separatist struggle for decades. It took a turn for the worse in January 2004, when Muslim militants seized an army arms depot making off with 300 weapons.

Besides the separatist struggle, the region - including Narathiwat, Pattani and Yala provinces bordering Malaysia - is rife with illegal activities such as drug smuggling operated by "influential people" who benefit from the state of fear and disorder.

The three provinces comprised the independent Islamic sultanate of Pattani more than 200 years ago before it fell under Bangkok's rule. More than 80 per cent of the three provinces' 2 million people are Muslim, making the region an anomaly in predominantly Buddhist Thailand. (dpa)