A Thai minister has admitted the country uses surveillance software to track individuals in cases involving national security or drugs, amid revelations that government critics' phones had been hacked using the Israeli-made Pegasus spyware.

Digital Economy and Society Minister Chaiwut Thanakamanusorn, said in Parliament late on Tuesday that he is aware of Thai authorities using spyware in "limited" cases but did not specify which government agency used such software, which programme was used or which individuals were targeted.

Human rights groups have accused successive Thai governments of using broad definitions of national security as a pretext to prosecute or suppress the activities of their main rivals.

A joint investigation by Thai human rights group iLaw, Southeast Asian internet watchdog Digital Reach and Toronto-based Citizen Lab highlighted on Monday the use of Pegasus spyware on at least 30 government critics between October 2020 to November 2021.

The probe followed a mass alert from Apple Inc. in November informing thousands of users of its iPhones, including in Thailand, that they were targets of "state-sponsored attackers".

Chaiwut did not name Pegasus but said that he is aware of spyware being used to "listen into or access a mobile phone to view the screen, monitor conversations and messages". But he added his ministry does not have the legal authority to use such software and did not specify which government agency does.

"It is used on national security or drug matters. If you need to arrest a drug dealer you have to listen in to find where the drop would be," he said.

"I understand that there was the usage of this sort but it is very limited and only in special cases."

His ministry has previously denied any knowledge of the matter.

The most recent alleged use of the spyware comes after the emergence of a youth-led movement in late 2020 that challenged the country's powerful monarchy and the government of Prime Minister Prayuth Chan-ocha. More than 1,800 people have faced security-related charges since the movement began.

Thai police in a statement denied the use of Pegasus for surveillance or breaches of privacy.

Pegasus has been used by governments to spy on journalists, activists, and dissidents and the Israeli firm behind it, NSO Group, has been sued by Apple and placed on a US trade blacklist.

NSO Group did not respond to Reuters' requests for comment.

- Reuters