STOCKHOLM (Reuters) -Sweden's security service has detained a Swedish diplomat on suspicion of espionage, public television SVT reported on Tuesday, citing unidentified sources.

The security service (SAPO) said it had detained a person on suspicion of spying after an operation in the Stockholm area in the last few days, but declined to give further information.

"It is correct that we have a case where the suspicion is spying," SAPO spokesperson Karin Lutz told Reuters. "One person has been taken into custody."

SVT said the suspect had been posted to a number of embassies around the world and that SAPO was investigating a potential connection between the suspect and the resignation of the government's national security adviser last week.

A security service spokesperson said SAPO had noted the media report but declined to comment further.

"The investigation is at an early stage and we cannot say anything due to secrecy considerations," the spokesperson said.

Tobias Thyberg, a foreign service veteran who in previous roles served as ambassador to both Ukraine and Afghanistan, resigned a day after being named as national security adviser.

Thyberg is not suspect in a crime, SVT said. Thyberg's predecessor stepped down in January and was then charged with negligent handling of classified information.

Swedish authorities have fretted in recent years about increasing threats from both foreign powers like Russia, China and Iran and groups engaging in actions ranging from violent attacks and hybrid warfare to corporate espionage.

In March, SAPO warned that foreign powers are operating in ways that threaten security, using hybrid activities to destabilise Sweden and Europe.

Justice Minister Gunnar Strommer told SVT the government had been informed of SAPO's operation and the person in custody was suspected "on reasonable grounds" of espionage. Reasonable grounds is the lower of two grades of suspicion in Sweden.

"The investigation has to be carried out and I don't want to preempt it," Strommer said in a statement to SVT.

Anton Strand, the lawyer appointed to defend the person in custody, declined to comment.

(Reporting by Simon Johnson and Johan Ahlander, editing by Terje Solsvik; editing by William Maclean and Mark Heinrich)