Denmark said on Tuesday it suspected an Iranian intelligence service had tried to carry out a plot to assassinate an Iranian Arab opposition figure on its soil.
A Norwegian citizen of Iranian background was arrested in Sweden on October 21 in connection with the plot and extradited to Denmark, Swedish security police said.
The attack was meant to target the leader of the Danish branch of the Arab Struggle Movement for the Liberation of Ahvaz (ASMLA), Danish intelligence chief Finn Borch Andersen said.
ASMLA seeks a separate state for ethnic Arabs in Iran's oil-rich southwestern province of Khuzestan.
"We are dealing with an Iranian intelligence agency planning an attack on Danish soil. Obviously, we can't and won't accept that," Andersen told a news conference.
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Iran's foreign ministry was not immediately available for comment on Tuesday, a public holiday in the Islamic Republic.
Andersen said the arrested Norwegian citizen had denied charges in court of helping a foreign intelligence service plot an assassination in Denmark.
Iranian Arabs are a minority in mainly ethnic Persian Iran, and some see themselves as under Persian occupation and want independence or autonomy.
Danish Foreign Minister Anders Samuelsen said on Twitter that the reported attack plot was "completely unacceptable".
"The government will respond to Iran and is speaking with European partners on further measurements," Samuelsen said.
On September 28, Danish police shut two major bridges to traffic and halted ferry services from Denmark to Sweden and Germany in a nationwide police operation to prevent a possible attack.
A few days earlier, the Norwegian suspect had been observed photographing and watching the Denmark home of the ASMLA leader, police said.
In November 2017, Ahmad Mola Nissi, an Iranian exile who established ASMLA, was shot dead in the Netherlands. The Danish security service then bolstered police protection of the ASMLA leader in Denmark and two associates.
Last month, Iran summoned the envoys of the Netherlands, Denmark and Great Britain over a September 22 shooting attack on a military parade in Khuzestan in which 25 people were killed.
Iran accused the three countries of harbouring Iranian opposition groups.
Another Arab opposition group, the Ahwaz National Resistance, and the Islamic State militant group both claimed responsibility for the parade attack, though neither has provided conclusive evidence to back up their claim.
Last week, diplomatic and security sources said France had expelled an Iranian diplomat over a failed plot to carry out a bomb attack on a Paris-area rally by an exiled Iranian opposition group.