The US has maintained its blockade of Iranian ports, while Iran has lifted and then re-imposed its blockade of the Strait of Hormuz, through which roughly ​a fifth of the world’s oil and liquefied natural gas passed ​before war broke out in the Middle East.

Amid ceasefire talks, Tehran, ⁠which controls the chokepoint, has proposed tolls on vessels to safely transit.

MARISKS on ​Monday issued an alert warning shipowners that unknown actors, claiming to represent Iranian ​authorities, had sent some shipping companies a message demanding transit fees in cryptocurrencies, Bitcoin or Tether, for “clearance”.

“These specific messages are a scam,” the firm said, adding the message was ​not sent by Iranian authorities.

There was no immediate comment from Tehran.

Hundreds of ​ships and about 20,000 seafarers remain stranded in the Gulf.

On April 18, when Iran ‌briefly opened ⁠the strait subject to checks, ships tried to pass, but at least two of them, including a tanker, reported that Iranian boats had fired shots at them, forcing the vessels to turn around.

MARISKS said that it believed that ​at least one ​of the vessels, ⁠which tried to exit the strait on Saturday and was hit by gunfire, was a victim of the fraud.

Reuters ​was not able to verify the information or track companies ​that ⁠had received the message.

“After providing the documents and assessing your eligibility by the Iranian Security Services, we will be able to determine the fee to be ⁠paid in ​cryptocurrency (BTC or USDT). Only then will your ​vessel be able to transit the strait unimpeded at the pre-agreed time,” said the message cited ​by MARISKS.