The Thai deputy prime minister dismissed on Monday a demand made by a Malay Muslim group to free those detained over alleged links to the long-running insurgency in Thailand's mainly Muslim south as a precondition for formal talks.

Officials of the Barisan Revolusi Nasional (BRN) met a Thai delegation at an undisclosed location in Southeast Asia on Friday and demanded the release of detainees, a leader of the group told Reuters in a rare interview.

The insurgency in the Malay-speaking region of the predominantly Buddhist country has killed some 7,000 people over the past 15 years and has flared on and off for decades.

"How can you say that? Everything must follow the justice procedure," Thai Deputy Prime Minister Prawit Wongsuwan told reporters on Monday when he was asked about the BRN's demand.

The BRN also demanded that the Thai government conduct a transparent investigation into alleged abuses by security forces after allegations that a man from the south, Abdullah Isamusa, 32, fell into a coma after being interrogated by the military.

The army said authorities were investigating and that there was no proof so far of torture.

The BRN, the most active insurgent group in the south, has opted to stay out of peace talks between the Thai government and other insurgent groups, although it said it held two previous meetings in recent years.

Yala, Pattani, and Narathiwat provinces were part of an independent Malay Muslim sultanate before Thailand annexed them in 1909.

- Reuters