Denmark has accused Iran's intelligence agencies of planning to assassinate an Iranian activist on Danish soil.
The target was believed to be a member of an Arab separatist movement, Denmark's intelligence service said.
Denmark has recalled its ambassador from Tehran and is consulting other EU countries about imposing new sanctions against Iran.
A massive police operation in Denmark last month linked to the case led to the bridge to Sweden being closed.
Iran has dismissed Denmark's allegations.
What is Iran accused of?
Danish Prime Minister Lars Lokke Rasmussen described the alleged planned assassination by Iran of an exiled separatist leader in Denmark as "totally unacceptable"
The Iranian ambassador to Copenhagen was summoned to the foreign ministry over the allegations.
A Norwegian citizen of Iranian origin was arrested in Sweden on 21 October in connection with the alleged plan. The man denies the charges.
Authorities conducted a massive manhunt on 28 September which led to road closures, trains and ferries being cancelled, and bridges being shut across Denmark.
On Tuesday, Danish intelligence chief Finn Borch Andersen confirmed the measures had been taken to prevent the alleged plot.
The alleged target was meant to be a senior member of the Arab Struggle Movement for the Liberation of Ahvaz (ASMLA), Mr Andersen said. The group seeks a separate state for ethnic Arabs in a region of western Iran.
The Norwegian suspect had been observed photographing the residence of the ASMLA leader, Danish security services said in a statement.
Following a deadly shooting at a military parade in Iran in September, in which more than 25 people were killed, Iran accused Denmark, the Netherlands and Britain of harbouring members of militant opposition groups.
That attack was claimed by a different Arab separatist group and the Islamic State group, although neither offered proof of their claims.
What has the response been?
An Iranian foreign ministry spokesman said such "biased reports" and allegations pursued "the enemy's plots and conspiracies" to harm the developing relations between Iran and Europe, according to Tasnim news agency.
Mr Rasmussen said, after a meeting with his British counterpart Theresa May in Oslo, that he appreciated her support.
"In close collaboration with UK and other countries we will stand up to Iran," he tweeted.
Foreign Minister Anders Samuelsen said Denmark would discuss further actions with European partners in the coming days.
The US Secretary of State, Mike Pompeo, congratulated Denmark on arresting "an Iranian regime assassin".
In August, Donald Trump re-imposed US economic sanctions on Iran, following the US withdrawal from the Iran nuclear deal earlier this year. EU leaders remain committed to the original agreement.
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On Friday, France expelled an Iranian diplomat over a bomb plot against a rally organised by Iranian opposition groups, diplomatic and security sources said.