One constant in US President Donald Trump's malleable foreign policy has been his fierce criticism of Iran and what he described as a weak and dangerous nuclear deal the US and other countries negotiated with Iran.

Threats and sanctions, and lots of them, have been his go-to response, lately leavened with vague offers of future negotiations.

Trump's reaction to attacks on two commercial tankers near the Strait of Hormuz may reveal the limits of his strategy of squeezing Iran's oil-dependent economy. "Iran did do it," Trump told Fox News, hours after the US miliary released footage it says shows a small Iranian ship sidling up to a damaged tanker and crew removing an unexploded mine. "You know they did it because you saw the boat. I guess one of the mines didn't explode and it's probably got essentially Iran written all over it," Trump said.

The tanker incident pushed tensions to a new height, with fears of a deliberate or accidental armed clash between US and Iranian forces.

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"I do not see Trump as worthy of any message exchange, and I do not have any reply for him, now or in future," Supreme Leader Ayatollah Ali Khamenei's website quoted him as saying in response to an offer of dialogue.

The Trump Administration has said its goal is to cut off all Iranian oil exports, humbling the clerical regime and potentially persuading it to trim support for terrorist proxy groups and open new negotiations. Each move by Trump — abandoning the nuclear deal negotiated by the Obama Administration, increasing sanctions and designating Iran's Islamic Revolutionary Guard Corps a foreign terrorist organisation — has pushed the US closer to a potential conflict.

Trump, who insists he wants to avoid Middle East wars, has to decide whether to reduce tensions or move ahead unilaterally and risk confrontation.

The Administration's critics say it is trying to provoke Iran to break the 2015 nuclear bargain Trump hates, since the deal did not collapse when Trump pulled out of it last year. But the "maximum pressure" campaign has not yet forced Iran to change its behaviour or come to the table for new talks. If anything, it has set up a contest with Iran that will make it hard for the regime to back down, analysts said.

"What (Ayatollah) Khamenei is saying to Trump is, 'You want to negotiate but you've made no offers and no concessions, and we will not respond to pressure'," said Barbara Slavin, of the Atlantic Council. "I think the Iranians are looking for some sort of gesture from the US. Otherwise, it's too huge a loss of face at this point to talk to Trump."

The US alone cannot enforce a full embargo on Iran, and Trump's campaign to reduce Iranian oil exports to zero has cost him leverage with close allies, some of whom are working to preserve the nuclear deal. Others suspect Trump or his advisers want conflict with Iran.

Iran has responded by threatening to start stockpiling low-level, nonweapons-grade uranium and to close off oil tanker traffic. US officials blame Iran for incidents including tanker attacks that appear to be an indirect show of force. Iran denies involvement

Foreign diplomats quietly point out that the US focus on ever-increasing sanctions has left it few friends outside the Middle East willing to back its Iran policy.