Iran is "in default" of the expectation that the nuclear agreement would improve regional security, according to Secretary of State Rex Tillerson.

"Iran is clearly in default of these expectations," Tillerson told reporters during a Thursday press conference.

Tillerson, who is in London to discuss the crisis in North Korea and Libya with British officials, maintained "no decision has been made" about whether President Trump will step back from the nuclear agreement. His comments reinforced the case for doing so, however, despite European opposition.

"[Iran is] threatening, not ensuring but threatening, the security of those in the region as well as the United States itself," Tillerson said. "So, we have to consider the totality of the behavior and not let our view be defined [solely] by the nuclear agreement."

That statement is the latest instance of U.S. diplomats arguing Trump has good cause to re-impose economic sanctions that were lifted in the context of the nuclear deal, even if Iran isn't technically violating the terms of the pact.

The prospect of such a move has alarmed European governments, who hope to do business in Iran. Trump has demonstrated a stronger affinity for the British government than some members of the European Union, but the United Kingdom is also lobbying for the U.S. to stay in the deal.

"It's so important that we make it work and we keep it alive," UK Foreign Secretary Boris Johnson said in the joint press conference.

Johnson agreed Iran should "stop being adventurous and expansionist and causing trouble in the region," but he argued the nuclear deal could induce the "young and potentially liberal" Iranian people to favor a more moderate government.

"I think it's important that they should see that there are benefits, economic benefits, from to the [nuclear deal] as well," Johnson said. "So, we in the UK want to keep that alive and that's certainly a point that we've been making to Rex and others in the U.S."

Tillerson made no such mention of hopes for Iranian reform.

"President Trump has made it clear to those of us who are helping him develop this policy that we must take into account the totality of Iranian threats, not just Iran's nuclear capabilities," he said.