The U.S. is raising pressure on European allies to isolate Iran in the wake of a foiled terrorist attack in France last month that Washington believes targeted American citizens, including former New York Mayor Rudy Giuliani.

Administration officials are citing the failed bombing in their effort to push European governments to choke off Iran’s access to everything from their airspace to cash deposits, arguing that Tehran’s involvement in terrorist plots obligate America’s allies to take a much harder line with the Islamic Republic. It was a message President Donald Trump delivered behind the scenes at this week’s NATO summit in Brussels, according to people familiar with the meetings.

"I would say there might be an escalation between us and the Iranians," Trump said at a news conference in Brussels on Thursday.

The alleged plot was scuttled two weeks ago after U.S. authorities tipped off European security officials of a plan to bomb the annual congress of the National Council of Resistance of Iran (NCRI), a controversial exile group that wants to oust Iran’s Islamic government.

Washington’s strategy for isolating Tehran goes beyond threatening sanctions against companies that do business in Iran.

Hundreds of American citizens, including Giuliani, who now serves as Trump’s personal lawyer, were among the thousands who attended the conference near Paris. Dozens of U.S. civilian and military officials, including Newt Gingrich, the former speaker of the U.S. House of Representatives, were also there.

Four people, including an Iranian diplomat, were arrested in connection with the suspected plot. Iran has accused NCRI of staging the attack in an attempt to discredit Tehran and damage its relations with Europe.

'Terrorism and proxy wars'

U.S. officials argue the French operation proves that the widespread view in European capitals of Iran as an honest broker is dangerously naïve.

"We ask our allies and partners to join our economic pressure campaign against Iran's regime," U.S. Secretary of State Mike Pompeo tweeted before meeting European allies to discuss Iran on Thursday.

"We must cut off all funding the regime uses to fund terrorism and proxy wars. There's no telling when Iran may try to foment terrorism, violence & instability in one of our countries next."

So far, the Americans have had little luck in winning over the Europeans. They have run into resistance from allies angered by Trump’s harsh criticism of Europe, which many fear is undermining the transatlantic relationship. European leaders are also still trying to salvage what they can of the Iran nuclear deal, which they argue was the West’s best chance of keeping Tehran from building a nuclear arsenal. The EU has even vowed to try to protect European companies against American sanctions.

Nonetheless, the U.S. campaign against Iran in Europe, which follows Trump’s May decision to pull out of the nuclear deal, suggests Washington’s strategy for isolating Tehran goes well beyond threatening sanctions against companies that do business there.

Earlier this month, U.S. Ambassador to Germany Richard Grenell sent a sharp-worded letter to the German foreign office, urging Berlin to bar Iran’s Mahan Air from operating in Germany. The U.S. has sanctioned the airline, which is closely linked to the Islamic Revolutionary Guard (Iran’s powerful paramilitary force), since 2011 and has repeatedly raised concerns over Mahan’s presence in Germany. Mahan, which the U.S. accuses of flying weapons and fighters into Syria and participating in terrorist activities, operates direct flights between Tehran and both Munich and Dusseldorf.

“We have provided evidence that Mahan Air has routinely flown fighters and materiel to Syria to support the [Bashar] Assad regime, actions which have contributed to mass atrocities in Syria and the displacement of millions across the region,” Grenell wrote in the letter, a copy of which was viewed by POLITICO. “As of yet, we have not seen any effort by the German government to shut down Mahan Air's operations in Germany or to press for EU sanctions against the airline.”

In addition to closing off Mahan’s access to Germany, a key commercial partner for Iran, the U.S. is pushing Berlin to reject an Iranian request to withdraw about €300 million in cash from the Hamburg-based European-Iranian Trade Bank. The planned Iranian withdrawal, first reported this week by Germany’s Bild, set off alarm bells in both Washington and Israel, where officials worry Tehran could use the cash to fund terrorist activities like the foiled French plot.

Exposing the details of that operation might be Washington’s best chance of getting Europe to take a tougher stance on Iran.

Strained relations with Europe

The alleged plot broke into view on June 30, the day of the NCRI convention, when two Iranian-born Belgian nationals, Nassim Nomeni and Amir Sadoni, both in their thirties, were arrested by Belgian police after a tip from U.S. intelligence. They were in possession of about half a kilo of explosives and a detonator, authorities say.

The man at the center of that operation, Assadollah Assadi, was arrested in Germany on July 2 while traveling on the autobahn in Bavaria. He is registered in Vienna as a diplomat, but U.S. and German authorities say he is an undercover intelligence operative.

Germany’s federal prosecutor charged him on Wednesday with commissioning the two Belgians to carry out an attack on the NCRI convention. Assadi is also suspected of providing the pair with the detonator. Authorities believe Assadi was trying to escape back to Iran after the Belgians were arrested. He is expected to be extradited to Belgium to stand trial.

U.S. officials have been urging their counterparts in Berlin and Brussels to accelerate investigations into the case. Many of the Iranian exiles present at the convention hold American citizenship.

The case has strained Iran’s relationship with important EU partners, in particular Germany and Austria. Assadi’s arrest came just two days before Iranian President Hassan Rouhani traveled to Vienna for a long-awaited visit that Tehran hoped would put its relationship with Europe on sounder footing.

“We expect full clarification in connection with this and I thank you, Mr. President, for assuring us that you will support this clarification,” Austrian Chancellor Sebastian Kurz said after meeting Rouhani on July 4.