Bangkok - A Royal Thai Air Force helicopter crashed Wednesday in the conflict-ridden deep South, killing all six people on board, officials said.

The Huey helicopter crashed shortly after departing from Betong border town in Yala province, 850 kilometres south of Bangkok.

"We still don't know what caused the crash," said Air Force spokesman Montol Sanchukorn. "We have sent a team to investigate."

It was the second helicopter crash in two months in the troubled deep South, which comprises the provinces Narathiwat, Pattani and Yala.

On June 20 an army helicopter crashed into a hillside in the Bannang Sata district of Yala province, 780 kilometres south of Bangkok, killing 10 out of 13 people on board.

The June 20 crash was blamed on an engine failure. The Thai military has more than 10,000 troops based in three-province region, where a separatist struggle has simmered on and off for almost a century but took a turn for the worse in January 2004 when Muslim militants attacked an army depot and stole 300 weapons, prompting a crackdown that further inflamed the local population against the government.

An estimated 2,700 people have died from an increasingly bloody separatist struggle in the region over the past three and a half years.

More than 80 per cent of the three provinces' 2 million people are Muslim, making the region an anomaly in predominantly Buddhist Thailand. (dpa)