A four-year-old girl and her father were among three killed Tuesday when a bomb hidden in a motorcycle's fuel tank exploded outside a school in Thailand's insurgency-plagued south.

The device went off as pupils and teachers filed into the school in the Tak Bai district of Narathiwat province for the start of the day.

Two of the dead were father and daughter, local police detective Noppdol Kingthong told AFP, adding that the bomb was set off by radio control as the pair neared the school gate.

A 23-year-old man later died in hospital and around a dozen other adults were being treated for injuries, according to a hospital worker who said the dead were Muslim.

Ethnic Malay insurgents in the kingdom's so-called ‘Deep South’ have for years targeted schools and teachers, which are seen as symbols of Thai state power in the culturally distinct Muslim-majority region.

More than 6,500 people -- the majority of them civilian -- have been killed by rebels and Thailand's Buddhist-majority security forces since 2004.

Debris from the blast was strewn across the area while an abandoned child's school bag lay on the road, according to an AFP journalist at the scene.

UNICEF's Thailand representative Thomas Davin said the agency was ‘shocked and saddened’ by the bombing.

‘No children nor any caretakers or education professionals should live or learn under fear of such attacks,’ he said in a statement.

- 'Aimed to kill' - Police and soldiers routinely accompany teachers and children to and from schools in the region's most dangerous ‘red zones’.

Dozens of teachers have been killed by rebels, some in front of pupils.

The rebels appear to have returned to attacking ‘soft’ civilian targets in recent weeks, following several months in which violence was aimed at security officials.

The bomb ‘aimed to kill... indiscriminately as shown by the four-year-old victim’, Colonel Pramote Prom-in, spokesman for the Thai army in the south, said in a statement.

The under-reported conflict in the southernmost provinces hit the headlines last month after rare bomb attacks in tourist hotspots inside the kingdom killed four people and wounded dozens, including foreigners.

Those blasts carried the hallmarks of the southern rebels, who never claim their attacks.

But Thai authorities have played down any possible expansion of the southern conflict, which has remained highly localised for more than a decade.

That is despite the identification of five suspects for the tourist attacks -- all Muslim men from the south, several of whom have a record of involvement in the insurgency.

On Friday the Thai junta held talks with a group that claims to represent the political aims of the shadowy rebels.

The aim is to prepare the ground for formal peace talks, which stalled following Thailand's 2014 coup.

But it is unclear if rebel representatives at the table have full authority over their foot soldiers.

The military is also widely distrusted by Malay Muslims in the south.

Rights groups says years of abuses by security forces including extrajudicial killings, have eroded faith in the Thai state.