By Agence France-Presse
The Muslim-majority border region has seethed with violence for over a decade as ethnic Malay insurgents battle the Buddhist-majority state for more autonomy.
Near-daily shooting and bomb attacks have claimed more than 6,800 lives since 2004, with both sides accused of rights abuses and atrocities.
In the latest violence Thursday an army patrol team was gunned down in Narathiwat province after their pickup truck was struck by a roadside explosive.
"After the car tipped over the road four (rangers) were shot dead at the scene," said provincial police commander Manas Suksamas.
Two other rangers died at the hospital, he added.
The three provinces that make up the 'Deep South' were colonised by Buddhist-majority Thailand over a century ago.
Thursday's attack comes a day before the anniversary of a bloody 2004 army raid on a mosque in neighbouring Pattani province that left 32 insurgents dead and stoked the rebellion in the region.
Militants often time their attacks to strike around anniversaries and other symbolic events.
Earlier this month the most active insurgent group rejected peace talks organised by the ruling junta in a rare press statement.
The shadowy Barisan Revolusi Nasional (BRN) is believed to be behind most of the violence in the region, although it never claims attacks and shuns publicity.
Experts have long said the faction is not loyal to a group of rebel negotiators who have been meeting with the Thai junta, which seized power in 2014.
The BRN called for international involvement in the army-led talks, which have so far borne little fruit.
But the Thai government has consistently refused to talk to the group.