A Swedish citizen was arrested accused of gathering information on refugees for overseas government consumption, Swedish intelligence agency Säpo announced.

The man, who has been under surveillance for some time, was arrested on Thursday in the Stockholm area, The Swedish Police Security Service (Säpo) said in a statement.

"In general it is illegal to systematically gather and pass on information about individuals in Sweden to a foreign power, either secretly or fraudulently," Säpo said. "This is called refugee espionage and is a crime that threatens national security."

The suspected man, who is a Swedish citizen, has been hired by a foreign embassy in Stockholm to spy on refugees here. According to Säpo, so called refugee espionage is a widespread serious problem in Sweden and is considered a threat to Sweden's national security.

"The crime is predicated on the involvement of a foreign power and mostly, though not always, the people in charge are very good at what they do”, prosecutor Tomas Lindstrand told the TT news agency, explaining that these cases are difficult to detect and difficult to investigate. “They take every possible precaution and I think I'd go as far as to say that they are considerably more careful than many people involved in more traditional serious crime."

The agency would not reveal any other details of the case, citing classified information, but said prosecutors must decide by Sunday whether to ask a court to remand him in custody.

If convicted, the man faces up to four years in prison.

Säpo said "several countries" were known to carry out refugee espionage in Sweden, but did not specify which ones.

"These states often invest major resources in mapping and persecuting regime critics and dissidents," the agency added.

"Refugee espionage not only targets refugees or asylum-seekers. Even people who live in Sweden and who are considered as dissidents or threats to regimes are affected."