Tensions between Tehran and Washington worsened significantly on September 4 as the US signalled it is not set to back a French plan to use $15bn of oil-guaranteed credit lines to assist Iran as a step towards bringing the Americans and Iranians together to resolve their differences.

A few hours after the US showed no enthusiasm for France’s plan, Iran’s president, Hassan Rouhani, said that his country would now expand its development work on new centrifuges for enriching uranium, in a third phased step away from compliance with the 2015 nuclear deal.

“We will take all necessary steps to protect the Iranian nation’s rights and interests ... Our third step involves the development of centrifuges. We will take this step on Friday,” Rouhani said in a televised speech.

The US, meanwhile, intensified its bid to throttle Iran’s economy—to force Tehran to agree to a far tougher renegotiated nuclear deal—by offering rewards for information that helps disrupt Iranian oil smuggling and assist the US in its plan to force crude oil exports from Iran to zero.

Iran has been gradually reducing its compliance with the nuclear deal, complaining that Europe has done next to nothing to protect its economy from sanctions despite being opposed to the US withdrawal from the accord and introduction of an ultra-aggressive sanctions regime. Prior to the US withdrawal in May 2018, UN atomic inspectors had no qualms about confirming Tehran was in full compliance with the terms of the agreement. Such compliance was supposed to protect Iran from the reintroduction of heavy international sanctions, but Donald Trump’s sanctions have proved heavier than anything the Islamic Republic has ever faced.

At the recent G7 summit in Biarritz, France, Trump suggested he might accept the French credit lines scheme if the US did not have to contribute. However, on September 4, the US State Department’s special representative for Iran, Brian Hook, called into question the very existence of the French proposal.

Asked about the initiative, Hook said: “There is no concrete proposal. We have no idea if there will be one. So we’re not going to comment on something that doesn’t exist.”

Hook then announced a ratcheting up of the Trump administration’s “maximum pressure” campaign against Iran, offering a $15m reward for anyone offering information that led to the disruption of oil smuggling. Washington claims such smuggling is being carried out by Iran’s Islamic Revolutionary Guard Corps (IRGC), the elite branch of the Iranian armed forces which the US, breaking all precedent, has listed as a terrorist organisation.

The state department also imposed sanctions on 16 entities and 10 individuals it accused of being part of an IRGC network smuggling oil to the Assad regime in Syria and Hezbullah in Lebanon, “and other terrorist actors”.

As he was making the announcement, the Financial Times reported that Hook had personally emailed the captain of a tanker carrying Iranian oil to offer him millions of dollars if he would steer the ship, the Grace 1 (lately renamed from Adrian Darya 1) to a country where it could be impounded.

The account was confirmed by the state department. The first email was reportedly sent 11 days after the ship was released by Gibraltar, where it was temporarily held on suspicion of shipping oil to Syria in breach of US sanctions.

“With this money you can have any life you wish and be well-off in old age,” Hook, the head of the state department’s Iran Action Group, purportedly emailed the ship’s captain, warning him: “If you choose not to take this easy path, life will be much harder for you.”

After the captain did not respond, he was apparently placed under US treasury sanctions. Hook has emailed or texted “roughly a dozen” captains in recent months to cajole or scare them out of helping Iran evade oil sanctions, according to the article.

The story was retweeted by Iran’s foreign minister, Mohammad Javad Zarif.

“Having failed at piracy, the US resorts to outright blackmail—deliver us Iran’s oil and receive several million dollars or be sanctioned yourself,” Zarif said. He has previously claimed he was threatened with sanctions if he did not go to the White House to meet Trump.

“Sounds very similar to the Oval Office invitation I received a few weeks back. It is becoming a pattern,” Zarif added.