Thailand is continuing a massive manhunt along the border with Burma following a gun attack on a school bus that killed three teenagers.

I have instructed the authorities to get them dead or alive... We will act decisively

Thaksin Shinawatra, Thai Prime Minister

Two children died at the scene on Tuesday morning, and a third died in hospital on Wednesday having suffered multiple gunshot wounds.

In addition to those killed, another 14 children were wounded, and some remain seriously ill in hospital.

The shooting by three unidentified gunmen took place in Ratchaburi province about 20 kilometres (12 miles) from the Burma border.

"I have instructed authorities to get them dead or alive, as authorities have asked for three days to arrest the gunmen," Thai Prime Minister Thaksin Shinawatra said.

"I assure you that people on Thai soil and doing bad things to the country cannot live here. We will act decisively".

The bus was sprayed with bullets

The border has been closed for the last week due to a series of clashes between Thai and Burmese troops, which started when skirmishes between Burma's military and armed rebels spilled over the border.

The BBC's Jonathan Head says there was no obvious motive for the bus attack.

The Burmese government has denied involvement in the shooting as has the main ethnic Karen militia group, the Karen National Union (KNU).

Mystery attackers

The attackers wore masks and jungle fatigues, similar to those worn by ethnic minority fighters.

However there are many different armed ethnic groups fighting in the region, including the Rangoon-allied United Wa State Army, which Thailand and the US accuse of drug-trafficking.

Mr Thaksin has said the evidence seemed to point to armed militias but said the KNU was unlikely to be involved because 80% of Thai children living in the area where the attack happened are Karen by descent.

On Wednesday more than 300 Thai police and soldiers backed by helicopters were combing a 100 square kilometre (39 sq mile) section of the border for the gunmen, a police spokesman said.

Thai Interior Minister Purachai Piemsomboon said he believed the assailants were still on Thai territory.

Three people, who were arrested on Tuesday, were released after they were confirmed to be government employees.

Meanwhile, about 500 Thai villagers living near the border were forced to flee their homes and move to a safer area inside the country after fighting erupted between the government forces and rebels in Burma.

'Slaughter'

Lieutenant Komdej Ruangsiri said footprints near the scene suggested the three attackers were backed by up to seven people.

Deputy government spokesman Kuthep Saikrachang said the Thai Government would retaliate against those behind the attack once they were identified.

Some of the wounded remain seriously ill in hospital.

Thai newspapers have reacted with horror to the shooting and called on Thailand and Burma to urgently settle their differences.

The Bangkok Post called the attack a "slaughter of the innocents" while the Nation said it was "one of the most heinous terrorist acts to have taken place on Thai soil in recent memory".

Ratchaburi province was the scene of a two-day hospital stand-off in January 2000 between Thai forces and rebels, in which all 10 rebels died.