In an apparent escalation of their tactics, suspected militants Saturday attacked a hotel in Hat Yai, the largest city in southern Thailand and a popular destination for tourists from neighbouring Malaysia and Singapore.
A car bomb in the basement triggered a fire which spread to a shopping mall within the Lee Gardens Plaza Hotel and killed three people, including a Malaysian tourist, according to the police.
Songkhla provincial governor Grisada Boorach said 416 people were injured, mostly suffering from smoke inhalation, and 140 were still in hospital Sunday.
Until now Hat Yai and Songkhla province have been relatively untouched by the shadowy insurgency that has claimed thousands of lives in the neighbouring Muslim-dominated provinces of Yala, Pattani and Narathiwat since 2004.
"There is no hint why they did this at this time," Hat Yai police chief Colonel Khomgrit Srisong told AFP by telephone. "We're questioning witnesses and the injured for more information."
The hotel bombing came about an hour after two car bombs minutes apart hit the town of Yala around midday as people were out shopping.
"Right now we know in Yala there are 11 people dead," a PR officer at the health ministry told AFP.
Colonel Pramote Promin, spokesman for the southern army region, earlier gave a toll of 10 dead and 117 wounded.
National police chief General Priewpan Damapong said the hotel bombing was linked to the Yala attack.
"It was a car bomb and it's related to the incident in Yala and I believe that it was the work of the same group," he said in televised remarks.
Colonel Pramote also said the attacks seemed similar.
"In the south there are not many insurgent groups who operate like this," he said on TNN24 television.
Prime Minister Yingluck Shinawatra said the security forces knew which group was responsible but she would not say in order to let them do their job.
A complex insurgency, without clearly stated aims, has plagued Thailand's far south near the border with Malaysia since 2004, claiming thousands of lives, both Buddhist and Muslim, with near-daily bomb or gun attacks.
The insurgents are not thought to be part of a global jihad movement but are instead rebelling against a long history of perceived discrimination against ethnic-Malay Muslims by successive Thai governments.
Struggling to quell the unrest, authorities have imposed emergency rule in the region, which rights campaigners say effectively gives the army legal immunity.
Human Rights Watch researcher Sunai Phasuk said the attackers were looking for maximum impact as Thailand prepares for the new year holiday in mid-April.
Sunai said such violent militants were a tiny minority among Muslims in the south "but they are ruthless, brutal, use violence and terror."
"The only way for the Thai authorities and Thai society to fight against these radical elements in the insurgent group is to make sure there are no conditions for them to justify their attacks," he told AFP, adding that the authorities must hold officials accountable for abuses in the south.
One of the region's deadliest incidents occurred on October 25, 2004, when seven people were shot dead as security forces broke up a protest in the town of Tak Bai, and 78 more suffocated or were crushed to death in trucks while being transported to a detention centre.
Rights groups have said the failure of Thai authorities to hold security forces to account over the deaths has fuelled further violence and alienation in the southern region.