US Secretary of State Marco Rubio said on Thursday a toll system in the Strait of Hormuz was unacceptable and would make a diplomatic deal unfeasible.
He also said “some progress” had been made in talks with Iran.
“We've always said a tolling system in the strait would be unacceptable. But we don't say that, the world says that,” Mr Rubio said as he was leaving the US for Sweden, where Nato foreign ministers are holding talks this week.
“It would make a diplomatic deal unfeasible. So it's a threat to the world that they would try to do that and it's completely illegal.”
Still, with regard to a potential deal with Iran, Mr Rubio said diplomacy remains a priority.
“We're going to do everything we can to see if we can get one [a deal]. If we can't get a good deal, President [Donald] Trump has been clear – he has other options,” he said.
He also said he would talk to Nato allies about some countries’ reluctance to allow the US to use its overseas airbases to bomb Iran.
“I know why Nato is good for Europe, but why is Nato good for America? Because of different bases in the region that allow it to project power during a contingency, in the Middle East or somewhere else,” he said.
“When that is the key rationale for why you’re in Nato, and then you have countries [like Spain] denying us the use of these spaces, well, then why are you in Nato? That’s a very fair question.”
He said he would discuss the issue with counterparts during talks in the southern Swedish city of Helsingborg on Friday.
Mr Trump has repeatedly blasted Nato after some members pushed back against US bases they host from being used to attack Iran. Spain refused, while other countries such as Britain were initially reluctant before backtracking.
The Strait of Hormuz has effectively been closed since the war started on February 28 and few ships have since passed through despite the ceasefire with Iran that started on April 7.
Some countries, particularly Britain and France, have offered to help reopen and police the strait, but they do not want to join any offensive operations, so efforts hinge on a durable ceasefire and political deal.
Mr Rubio and the State Department, meanwhile, are pushing for a diplomatic effort, called the Maritime Freedom Construct, that would form a coalition to help deal with the strait.
