A policeman's wife was shot dead and then set on fire in front of terrified shoppers at a busy market in Thailand's violence-ridden deep south, officials said Monday, in an apparent revenge attack.

The 28-year-old Buddhist woman was gunned down on Sunday afternoon as she returned to her car from a market in the Ratapanyang area of Pattani province.

A local police officer said a note left at the scene indicated the attack was in revenge for the deaths last week of three Muslim brothers -- aged three, five and nine.

Insurgents blame Thai authorities for the boys' murder -- although there has been no official evidence to back up the claim.

The latest killings raise the spectre of a spiral of tit-for-tat killings between Muslims and Buddhists in Pattani.

The restive region is one of three Muslim-majority provinces in the grip of a bloody decade-long insurgency that has claimed over 5,900 lives -- the majority of them civilians.

After being shot, the woman's body was torched in front of market goers, the police officer told AFP, but no-one had come forward with information fearing retribution from the attackers -- who are suspected militants.

Last Monday, the Muslim brothers were gunned down in front of their home in neighbouring Narathiwat province. Their pregnant mother and father were also shot in the attack but survived.

"We will keep killing you as long as you are still on our land," the letter said, according to the officer who added Sunday's victim was "an unarmed, soft target from the Buddhist community".

Srisompob Jitpiromsri, at Prince of Songkla University in Pattani, said the boys' deaths "have set off a chain-reaction which will be hard to control unless authorities can bring to justice their killers".

"The insurgent movement is taking their deaths as an opportunity for revenge. Local feelings over this are running very high," he said.

The insurgents want a level of autonomy from Thailand, which annexed the region over a century ago.

They accuse Thai authorities of riding roughshod over the local Melayu culture as well as widespread human rights abuses.