The US is calling on the UK and European states to take a tougher stance to hold Iran accountable for its terror-linked activities.
Tensions between Washington and Tehran escalated last year following the US imposition of economic sanctions and its decision to withdraw from the 2015 Iran nuclear deal.
Nathan Sales, the US ambassador-at-large and coordinator for counter-terrorism, said in an interview with The Daily Telegraph that the UK will likely be targeted next as it is a "fertile ground" for Iran's terror campaign.
“If there are no costs, Iran is going to keep at it. So it's incumbent on us to impose those costs so that we can deter future acts of terrorism,” he said.
The US official blamed Iran for a recent foiled bomb attack that targeted a political opposition rally in Paris and an alleged plot to murder an exiled political leader in Denmark.
"Terrorism is fundamental to the Iranian regime raison d'etre. They regard the export of their revolution as absolutely fundamental and central to the regime's identity," he said.
Mr Sales hailed the expulsion of Iranian ambassadors from European states in the early 1990s following a bomb attack by stating the move is likely to be effective if similar steps are taken now.
He expressed concern at the number of attempted terror plots that Tehran has allegedly carried out in Europe.
The anti-Iran rhetoric comes in line with US President Donald Trump’s efforts to rally support behind his strategy to increase pressure on Tehran as a means to change its behaviour.
Mr Sales welcomed the UK’s decision to list Lebanese militant group Hezbollah as a terrorist organisation.
He urged EU states to do the same.
“What we need now is the same sort of robust, assertive response to signal to Tehran this is unacceptable and if you do it, we’re going to make you pay a price.”
Washington designated the group as a foreign terror organisation in 1997.
Tehran is a key supporter of Hezbollah, US intelligence sources said last October that Iran was supplying Hezbollah with Global Positioning System (GPS) components to make previously unguided rockets into precision guided-missiles.
“Hezbollah is one organisation. Its leaders, its members, do not differentiate between their military terrorist activities on the one hand and their so-called political activities on the other,” he said.
Hezbollah was established in 1982 during Lebanon’s civil war and is now a major political party in the country, holding three cabinet posts.
US Secretary of State Mike Pompeo said the Iran-backed group has "American blood on its hands" and continues to plot and carry out attacks in the Middle East, Europe, and around the world.
On the issue of ISIS foreign fighters captured in Syria, Mr Sales, who is involved in the negotiations, said that Washington is demanding that the UK and Europe allow the individuals to be prosecuted in their own courts.
Yet, the UK is stripping the fighters from their British citizenship and is not allowing them to return home.
“In the United States but more broadly European and other advanced democracies, civilian courts have proven themselves to be entirely capable of meeting this challenge."