The US State Department has confirmed one of its officials offered millions of dollars in cash to the captain of an Iranian tanker to allow the seizure of the vessel.
The head of the department's Iran Action Group, Brian Hook, reportedly emailed the captain of the Adrian Darya 1 about sailing the ship to a port where the US could seize it.
The ship, previously known as the Grace 1, has been at the centre of a diplomatic row after it was detained by Royal Marines off the coast of Gibraltar in July.
It was impounded for six weeks, amid suspicions it was carrying crude oil to Syria in breach of EU sanctions.
According to the Financial Times, Mr Hook emailed the Indian captain of the ship, Akhilesh Kumar, with "good news".
“With this money, you can have any life you wish and be well-off in old age," Mr Hook's email read.
“If you choose not to take this easy path, life will be much harder for you."
The newspaper reports Mr Kumar did not reply.
The US State Department confirmed the accounts as "accurate": "We have seen the Financial Times article and can confirm that the details are accurate.
"We have conducted extensive outreach to several ship captains as well as shipping companies warning them of the consequences of providing support to a foreign terrorist organisation."
The US classifies Iran's Revolutionary Guard as a terrorist organisation.
On Wednesday, the Trump administration announced new sanctions had been put on an Iranian shipping network used to sell oil.
A $15m reward was offered to anyone who could disrupt the scheme, said to be controlled by Iran's Revolutionary Guard.
Mr Hook told the Financial Times that the US is "working very closely with the maritime community to disrupt and deter illicit oil exports".
On Twitter, Iran's foreign minister Javad Zarif accused the US of "outright blackmail".
The ship was reflagged as the Adrian Darya 1 following its release by Gibraltarian authorities on 15 August after Iran gave assurances about its destination.
The US attempted to block the release of the vessel and, days later, issued a warrant to seize it.
Last Friday, the US blacklisted the tanker, claiming it was transporting 2.1 million barrels of crude oil to Syria.
On Tuesday, the Adrian Darya 1 turned off its tracking beacon with its last location said to be 83 kilometres off the coast of Lebanon and Syria.
Tensions have been rising between the US and Iran since President Donald Trump pulled Washington out of the 2015 nuclear deal and reimposed sanctions.