Police have for the first time claimed that "some Thais, mainly in the far South", have links and provide support to the Islamic State (IS) group.
The announcement came after Australian police informed Thai authorities that some groups of Thais were providing financial support to the IS.
Australian police also found that over 100,000 Facebook users from Thailand visited IS-related online communities over the past year.
Thai authorities had previously consistently denied any connections between Thais and the IS.
- See also: IS supporters 'travelled to Syria'
Until now, insurgents in the South have refused to establish any links with foreign pro-terrorist groups, including al-Qaeda and the Jemaah Islamiyah group.
While Malaysian, Indonesia and Singaporean nationals have been identified inside Islamic State's so-called "caliphate", no Thai ever has been shown or identified as linked to the IS group. Tuesday's claim of links between southern Thais and Islamic State surprised most security experts in Bangkok.
Deputy national police chief Srivara Ransibrahmanakul said the information provided by Australian police was part of a probe which had linked some Thais to the radical Islamist group.
Australia has not commented publicly on the claim by Thai police.
But the deputy national police chief insisted Australians had informed Thai authorities that "some Thais" have frequently travelled back and forth between Thailand and Syria.
It was determined that those people lived in areas under the jurisdiction of Provincial Police Region 9 which covers the southern provinces of Songkhla, Satun, Trang and Phatthalung, and the Southern Border Provinces Police Operation Centre which is responsible for the southern border provinces of Pattani, Narathiwat and Yala, Pol Gen Srivara said.
He did not say how it had been determined. But Pol Gen Srivara said police from the Central Investigation Bureau and the Special Branch Police have been instructed to investigate the matter and will report the findings to the government.
He did not say who issued the order for an investigation.
Pol Gen Srivara added police have been analysing the user names of the Facebook users which are being divided into six groups. He declined to reveal further details, only saying some of them may be IS sympathisers.
Prime Minister Prayut Chan-o-cha on Tuesday said authorities were identifying the suspects and were prepared to take preventive measures and arrest anyone involved.
"The whole world is affected by the IS threat. Every country is facing this problem," he said.
Deputy Prime Minister and Defence Minister Prawit Wongsuwon said the Special Branch Police and Immigration Bureau have been ordered to take precautions, adding Asean countries already have a cooperation agreement to counter terrorism.
"Those Thais may just want to follow IS information without giving financial support to the group," said Gen Prawit.
Meanwhile, Apichart Suriboonya, commander of the Royal Thai Police's foreign affairs division yesterday spoke at the UN General Assembly, representing Thailand and the Royal Thai Police.
Expressing support for a draft resolution on cooperation between the UN and the International Criminal Police Organisation (Interpol), Pol Maj Gen Apichart told the gathering that Thailand and the Royal Thai Police had been asked by Interpol to join Switzerland, Rwanda, Jordan and Jamaica to support the resolution on fighting trans-national crime.
Pol Maj Gen Apichart said that transnational crime and terrorism are expanding rapidly in an increasingly connected world, and stressed that combating these threats should not be restricted to police and state security officials.
It is important to enlist cooperation from the civil sector and the private sector to boost efforts in tackling these challenges, he said, adding that intelligence operations will support these efforts.
He also said the UN should put in place mechanisms to channel information to combat transnational crime and terrorism directly to authorities at operational levels.
This information should not be left in the hands of high-level officials for too long as a quick response to these threats is necessary so as to keep any damage to a minimum, he said.
He added Interpol has such a mechanism -- a communications network which covers 190 countries and is being extended to reach operational-level officials in several countries, including Thailand.
With this network, Thai police have arrested up to 55 transnational crime suspects wanted by Interpol over the past three years, Pol Maj Gen Apichairt said.
The 193 member states of the UN later unanimously voted in support of the draft resolution.
After the vote, Pol Maj Gen Apichart said the communications network will become more formal and will serve as the main channel for UN member states to share intelligence to combat transnational crime and terrorism.
