Marco Rubio said there are "some good signs" that a deal to end the Iran war could be in sight - but added that he doesn't "want to be overly optimistic".
The US secretary of state added that while the talks are progressing - with Pakistani representatives travelling to Iran on Thursday - Washington is dealing with "a system that itself is a little fractured".
Mr Rubio added that any diplomatic deal would be "unfeasible" if Iran imposes a toll on the Strait of Hormuz.
"No one in the world is in favour of the tolling system," he said. "It can't happen. It would be unacceptable.
"It would make a diplomatic deal unfeasible if they were to continue to pursue that. So it's a threat to the world if they were trying to do that, and it's completely illegal."
Mr Rubio was speaking to reporters in Florida on Thursday as he boarded a flight to Sweden, where he will meet with NATO foreign ministers.
The Strait of Hormuz, which is a conduit for one-fifth of global oil exports, has effectively been blocked by Tehran since the US and Israel launched strikes on Iran on 28 February. The stand-off escalated last month when the US began blocking Iranian ports.
Iran has claimed sovereignty over the strait, saying it will only fully reopen the waterway if it gets to charge commercial vessels tolls for passing through.
The disruption has had a wide impact on global economies, sending energy prices soaring and hitting government borrowing.
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A shaky US-Iran ceasefire has been in place since early April, but both the Trump administration and Iranian officials have repeatedly threatened to resume attacks.
Donald Trump said in a Truth Social post earlier this week that he had told the US military "to be prepared to go forward with a full, large-scale assault of Iran, on a moment's notice" if a deal isn't reached.
The US president echoed his previous comments as he spoke at the White House on Thursday, telling reporters he will "have to do something very drastic" if Iran refuses to guarantee it will not acquire a nuclear weapon.
Meanwhile, Iran's Revolutionary Guard threatened that it would "extend the war beyond the region" if the US attacks Iran again.
(c) Sky News 2026: 'Some good signs' a US-Iran deal to end war could be in sight, Marco Rubio reveals
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