A string of bomb attacks targeting Thailand´s crucial tourism industry have killed four people, authorities said Friday, sending authorities scrambling to identify a motive and find the perpetrators.

Prime Minister Prayut Chan-O-Cha called for calm and said he did not know who was behind the attacks.

Twin bombs exploded in the upscale resort of Hua Hin late Thursday, killing one woman and wounding more than 20 others, including nine foreign tourists, and were followed by two more on Friday morning that killed another person.

AFP journalists saw injured and bleeding people lying on the pavements shortly after the latest explosions, as emergency workers rushed to the scene and took them away in ambulances.

A further two blasts struck Friday at Patong Beach on the popular tourist island of Phuket, while two more bombs were reported in the southern towns of Trang and Surat Thani, in each of which one person was killed.