Another officer was injured when the bomb exploded at about 6.40am at a railway bridge in Songkhla's Chana district. Police blamed the blast on suspected Islamic militants.

The railway attack came just hours after a string of about 70 coordinated bomb and arson attacks in Narathiwat, Pattani and Yala in late Tuesday that injured at least three people.

The violence on Tuesday targeted the homes of police and government officials, karaoke bars and a train station. Police had defused more than 20 bombs laid on the southern train track Tuesday night,

The three provinces have been under emergency decree rule since October 2005, allowing security authorities to arrest suspects without charges, impose bans on public gatherings and other draconian measures.