The Singaporean man who spied on the United States for China was arrested by the Internal Security Department (ISD) yesterday upon his return to Singapore.

Dickson Yeo Jun Wei, 39, was sentenced to 14 months' jail in the US for spying. He had revealed to investigators there that his "previous intelligence taskings targeted other states apart from the US", said the ISD in a release yesterday.

The ISD will interview Yeo to establish if he had engaged in "activities prejudicial to Singapore's security".

"Singapore will not allow our nationals to be subverted or used by any foreign actors for activities prejudicial to our security and national interests," said the department.

"The Government takes a very serious view of any Singaporean who enters into a clandestine relationship with a foreign government and engages in espionage or subversive activities at the behest of the foreign power," it said.

The ISD added that such individuals will be firmly dealt with.

It cannot provide any further information as investigations are still under way.

The ISD also said that precautionary measures are in place to ensure there is early detection, infection control and safe distancing in the facility where Yeo is being held.

"Precautionary measures were taken during the arrest process to safeguard the health of personnel involved," said the department, adding that Yeo was still subject to the "prevailing border control requirements" when he arrived in Singapore.

In July, Yeo pleaded guilty to obtaining sensitive information from Americans under the direction of Chinese intelligence officials.

He had maintained that he was supportive of the Chinese "cause", even towards the end of his court hearings and he claimed that he did not "betray Singapore".

He spied on the US for four years, after being recruited by Chinese intelligence officials in 2015. At that time, he had gone to Beijing for a presentation on the political situation in South-east Asia.

Yeo was then a PhD student at the Lee Kuan Yew School of Public Policy. He was tasked by Chinese intelligence to spot and assess American citizens, including US military and government employees, who had access to valuable non-public information.

Yeo admitted that he paid American citizens to write reports that he then sent to the Chinese government without the writers' knowledge.

He set up a fake political consultancy to do so and trawled through social networking sites to find his targets.

He was caught and arrested in the US in November last year.

After news broke of Yeo's arrest, China denied recruiting him as a spy and accused the US of having reached "a state of extreme suspicion".

Yeo's PhD candidature was terminated when the school learnt of his espionage activities.

The ISD said it has informed Yeo's family members of his arrest and that the department is in contact with them, should they need any assistance.

However, as it is still investigating Yeo, he may be denied calls and visits, said the ISD.