Thai police say a bomb blast in southern Narthiwat province has injured at least 15 people.

The police say they do not yet know whether anyone was killed in the explosion, which occurred Thursday in the town of Sungai Kolok.

It was not immediately clear who was responsible. But a Muslim separatist group called the Pattani United Liberation Organization had warned of a violent backlash over the deaths of some 84 people during protests Monday in southern Thailand.

Thai Prime Minister Thaksin Shinawatra has promised an investigation into the incident that brought widespread international outrage, particularly in majority Muslim countries

Most of the deaths were attributed to suffocation after police packed detained protesters in a truck for transport to a detention center.

Some information for this story provided by AP and Reuters.