Tankers that want to pass through the Strait of Hormuz are being advised not to pay money to Iran to allow them through, after a ceasefire agreed on Tuesday failed to get traffic moving through the waterway again.
The agreement was supposed to include reopening the strait but Iran has suggested that ships must seek its permission or they may still be "targeted and destroyed" and has said it could levy a fee in return for safe passage.
"We do not believe the payment of tolls is the right way to go about this," said Phil Belcher, from Intertanko, a group representing tanker firms.
"We are amazed that this appears to be one of the starting points of negotiations," he told the BBC.
US vice president JD Vance is meeting representatives of the Iranian government in Islamabad in Pakistan on Saturday to try to nail down details of a ceasefire deal that already appears to be in jeopardy after continued air strikes in Israel and Lebanon and a stalemate over the vital shipping lane.
