Iranian authorities have detained a Swedish man, Stockholm said Friday, threatening to strain already tense ties between the countries.

Sweden's foreign ministry confirmed to AFP that a Swedish man in his 30s "has recently been detained".

"The embassy in Tehran is seeking further information in the matter and is in contact with local authorities," it said in an email, providing no further details.

Swedish newspaper Aftonbladet reported the man was on holiday together with other Swedes when he was detained.

Relations between Sweden and Iran are already tense over a trial in Stockholm against an Iranian ex-prison official accused of war crimes over his alleged role in a 1988 purge of dissidents.

Hamid Noury, 61, was arrested at a Stockholm airport in November 2019 after Iranian dissidents in Sweden filed police complaints against him.

The trial ended on Wednesday with the court due to issue a verdict in July.

Earlier this week, Iran's foreign ministry demanded Noury's release, saying it considered his arrest and trial "illegal".

Meanwhile, the Iranian news agency Isna reported this week that Iran plans to execute Swedish-Iranian academic Ahmadreza Djalali, who was sentenced to death in 2017 for spying for Israel, before May 21.

The Swedish foreign ministry last week advised its nationals against non-essential travel to Iran.