Malaysia has confirmed the arrest of a man described by Thai Prime Minister Thaksin Shinawatra as the mastermind behind an upsurge in Muslim separatist violence in southern Thailand.

Deputy Internal Security Minister Noh Omar said the man was arrested on Jan 5 and was being held under the Internal Security Act, which allows for indefinite detention without trial.

He identified him as Abdul Rahman Ahmad, also known as Deraman Koteh. Thaksin named him as Doramae Kuteh, also known as Chae Kumae Kuteh, and said checks were being made on his nationality.

Noh declined to say whether the man under arrest was suspected of being the leader of Muslim separatists accused of a resurgence of violence in mainly Buddhist Thailand which has left at least 570 people dead in the past year.

"Everything is being investigated," Noh was quoted as saying by local media today.

No extradition

Prime Minister Abdullah Ahmad Badawi said today that the detained man was a Malaysian and was not believed to hold dual nationality.

"On the basis of whatever papers that were available on him and whatever we can get from his house, there is no other evidence to show that he is a Thai. But he has documents which prove he is a Malaysian citizen," Abdullah told a news conference.

"We have no extradition (treaty) with Thailand but if Thai authorities want to seek some help from us in whatever way, (if) they want to know the activities of this man, we will be able to cooperate," he said. "But we have to wait for the formal request from Thailand."

Thaksin said today if the man were found to hold Thai citizenship, Thailand would seek his extradition.

"We are checking whether he has Thai nationality or not, and if he has Thai nationality we will seek his extradition," Thaksin told reporters.

The arrest again underscored tensions between the neighbors over the insurgency.

Diplomatic channels

Malaysia yesterday criticised Thaksin for telling reporters that he wanted the alleged militant extradited from Malaysia, saying he should have followed diplomatic channels.

"This is not the way to cooperate with each other," Foreign Minister Syed Hamid Albar told a news conference.

"Whatever it is that needs to be done, there are proper channels between Malaysia and Thailand," Syed Hamid said testily.

The Thai foreign ministry said it was "disappointed with the reaction expressed by the Malaysian foreign minister".

"Furthermore, it is deemed inappropriate for a person holding ministerial rank of a neighbouring country to make negative statements directed at the leader of another country in such a manner which is not constructive to their existing close relations," the ministry said in a statement.

This is not the first time mainly Muslim Malaysia has been irritated by Thaksin's remarks about the alleged cross-border movements of Thai Muslim militants, many of whom share a kinship with Malays in Malaysia's northern states.

Last month Abdullah denied that Thai Muslim insurgents were being trained in Malaysia and chided the Thai premier for making such "sensational" claims.

Abdullah said he was disappointed Thaksin did not use diplomatic channels to air his allegations.

"I was shocked to read about it. If Thaksin had such information and we don't know about it, he should have passed it over to us officially through proper diplomatic channels," Abdullah said.