"They beheaded one victim and left a message on the wall saying 'You kill the innocent and we kill Thai Buddhists'," he told reporters.
Attacks on foreign workers have been rare in Yala, Pattani and Narathiwat, the three Malay-speaking southern provinces where 2,000 people have been killed since an Islamic insurgency flared in January 2004.
In Yala, attackers shot and killed a 41-year-old Buddhist man on Monday morning while he was working at his rubber plantation, police Lieutenant Colonel Jaras Chinaphong, said.
Also on Monday, a homemade bomb triggered by a mobile phone exploded in a crowded market in Yala, injuring seven people, said police Colonel Phoompet Pipatpetpoom.
Australia has issued two travel alerts in recent weeks saying intelligence pointed to a high threat of bomb attacks.
It said the increase in violence in southern Thailand might lead to attacks elsewhere in the country, including the nation's capital Bangkok and tourist areas such as Phuket and Pattaya.
Sonthi Boonyaratglin, Thailand's army chief, said on Sunday he did not expect attacks in Bangkok although security forces were placed on alert.
On New Year's Eve, a series of bombs killed three people in Bangkok.