BANGKOK—Not far from the world-famous tourist beaches of Phuket and Krabi, a violent conflict in southern Thailand appears to be entering a dangerous new phase as Islamist insurgents expand their attacks against local Buddhists to include outsiders entering the Muslim-majority region.

The violence in southern Thailand has left more than 5,000 people dead since the conflict erupted in 2004. The latest attacks came Sunday, when suspected militants killed five soldiers and wounded several more in a pair of ambushes, police said....