"There was a clash between militants and rangers this morning. The clash killed four militants and one ranger," a policeman at Janae district in the southern province of Narathiwat told AFP, asking not to be named.
More than 6,500 people have died -- the majority civilians -- since 2004 in fighting between Malay Muslim rebels and Thai troops and police who blanket the area.
Thailand annexed the culturally distinct zone bordering Malaysia over a century ago.
Bombings, drive-by shootings and executions occur weekly in the region, with little progress made in peace talks since the military seized power in Thailand in a coup two years ago.
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The insurgents are seeking greater autonomy from majority-Buddhist Thailand, which they complain has ridden roughshod over their culture and language.
The rebels employ brutal tactics including shootings, beheadings and bombings, often targeting perceived civilian collaborators such as teachers and Buddhist monks.
In April insurgents briefly seized a hospital and used it to launch an attack on nearby troops during a night of multiple coordinated assaults.
The move sparked condemnation from the UN and a vow by Thailand's junta to tighten security in the region.
The military also stands accused of routinely abusing human rights including torture and extrajudicial killings.
Rights groups say peace is unlikely while a tight security net remains over the region.
Critics also cast doubt on the army's sincerity and the ability of their rebel interlocutors to control the revolt's foot soldiers.