Deputy Prime Minister Prawit Wongsuwon expressed concern about the risk of Islamic State (IS) fighters and members of other terrorist organisations sneaking into Thailand.
Gen Prawit made the remark during an interview about the current crackdown on transnational criminals who may be among the approximately 100,000 foreigners believed to have overstayed their visas.
Defence Ministry spokesman Kongcheep Tantravanich provided this figure on Sunday.
The deputy premier, who is in charge of security affairs, said the government now fears there “probably” are elements of foreign terrorist groups in the country.
He also denied speculation that foreigners whose visas have expired can remain in Thailand by bribing security officials.
He even urged prospective whistle-blowers with information related to bribe-taking to step forward so that probes into any allegations of misconduct could be launched.
Over 100 people with expired visas have already been arrested recently in the South, Gen Prawit said.
Pol Maj Gen Itthiphol Itthisanronnachai, deputy chief of the Immigration Bureau, said over 100 people have been detained in Phuket alone.
He acknowledged the scale of visa abuse creates a window of opportunity for dishonest state officials to extort money from offenders.
As most of those who overstay their visas have been found to reside in tourist areas, he urged operators of accommodation services to keep an eye out for any “irregularities” and report them to the authorities.
This could be foreigners who stay for a long time without appearing to have a job or any form of legitimate income, he said.
Such cases should be reported to the police so that formal checks can be carried out, he added.
Lt Gen Kongcheep said security officers have tightened campaigns to investigate and arrest transnational criminals after various tip-offs from the public.
Raids have been carried out in the Nana, Phra Khanong, On Nut and Ramkhamhaeng areas of Bangkok as well as border locations and major provinces and cities.
Pol Maj Gen Surachate Hakparn, acting deputy commissioner of the Tourist Police Bureau (TPB), earlier said operations against transnational crimes would continue and would target those who had overstayed their visas.
The move, he said, is in line with policies touted by the government and national police chief Chakthip Chaijinda, who want to ensure public safety and boost tourist confidence.
A large number of foreigners were found to have overstayed their visas, particularly Africans, he said, noting that overstayers often posed as language teachers, priests or footballers.
Some do not have proper jobs but have between 500,000 baht and one million baht wired into their bank accounts regularly, he said earlier. They are likely involved in drugs and various kinds of scams, he said.