The mid-morning attack was a grizzly reminder that rebels in the Muslim-majority deep south have yet to curb violence against Thai security forces -- or civilians -- despite ongoing peace talks in neighbouring Malaysia.
Police said the rangers were travelling in a pick-up truck to meet Muslim community leaders in the Saiburi district of Pattani, one of Thailand's southernmost provinces hit by a near-decade long rebellion which has claimed more than 5,500 lives.
"Five rangers are dead, including the commander who was intially severely wounded," Sergeant Montri Prommee of Saiburi police told AFP, updating the death toll from four and adding the explosive device was buried in the road.
"They (the insurgents) want to create situations (unrest) on important days," he added, referring to the timing of the attack on one of the most important Buddhist holidays of the Thai calendar.
Analysts say rebels are using increasingly sophisticated bomb-making and detonation techniques to cause more casualties.
On Thursday another ranger was killed alongside a suspected militant in a late-night shootout in Narathiwat, which borders Malaysia, while a Buddhist grocer was gunned down in broad daylight the previous day.
This week's bloodshed follows the first official peace talks between Thai authorities and representatives of rebel group the Barisan Revolusi Nasional (BRN) in Malaysia in March and another round in April.
Since then, near-daily deadly attacks have renewed questions over whether Thailand is talking to rebel leaders who can control the violence.
Buddhist and Muslims alike fall victim to the shadowy militants, who target security forces, civilians and perceived representatives of state authority such as teachers.
In April rebels involved in the talks said they wanted "liberation" from Thailand, something powerful Thai army chief General Prayut rejected on Wednesday during a trip to the southern province of Yala.