Copenhagen recalled its ambassador to Tehran for consultations after Danish intelligence accused Iran of planning an attack in Denmark against three Iranians -- an allegation denied by the Islamic republic.

Danish Prime Minister Lars Lokke Rasmussen said it was "totally unacceptable" for Iran to plan such an attack, while US Secretary of State Mike Pompeo congratulated Denmark on arresting what he called "an Iranian regime assassin".

But there was a more measured response in Brussels, where officials are desperately trying to save the 2015 agreement to curb Iran's nuclear programme after Washington abruptly pulled out.

"We deplore any threat to EU security and take every incident extremely seriously, and therefore we stand in solidarity with the member state concerned, in this case Denmark," Maja Kocijancic, a spokeswoman for the European Commission, told reporters.

US President Donald Trump pulled the US out of the Iran accord -- known officially as the Joint Comprehensive Plan of Action (JCPOA) -- in May, slamming it as a bad deal and reimposing punishing sanctions on Iran.

The EU and the three European signatories to the deal -- Britain, France and Germany -- have been trying to keep the deal alive.

Kocijancic insisted efforts to save the deal did not mean the EU was overlooking other issues such as Iran's ballistic missile programme and interference in regional conflicts.

"But we believe that the JCPOA, which has a very clear aim, needs to remain in place because it is an element of security, while other issues need to be addressed with all seriousness in parallel," she said.