European tanker owners are already proving wary of shipping Iranian crude in the light of new US sanctions.
Industry sources cited by the Platts news agency said companies fear secondary action from the US after it pulled out of the joint nuclear deal.
Overseas buyers of Iranian oil must wind down contracts by 4 November, according to a US Treasury Department fact sheet.
The London P&I Club, however, warned it had been notified by the US treasury department that penalties could be imposed before this.
One Greek shipowner told Platts: "We will have to avoid calling at Iranian ports for now as it has been mooted that sanctions may be applicable on any contracts made after 8 May, so we are erring on side of caution and avoiding Iranian ports until there is more clarity on the situation."
Polembros Shipping told TradeWinds: "Our understanding is that all charterers and owners are waiting for further news from their P&Is who are in turn waiting to hear back from OFAC [US Office of Foreign Assets Control]."
Maersk Tankers said last week it would honour customer agreements entered into before 8 May, but then wind them down by 4 November "as required by the reimposed US sanctions", Business Day reported.
The owner said it "has been transporting cargoes for customers in and out of Iran on a limited basis."
Buyers are already seeking Russian, Saudi Arabian and Iraqi crude as alternatives.
"Today it looks like many June lifters will have to be cancelled due to not being able to fix ships," a crude oil trader active in the Mediterranean market said. "We saw the Greeks and the Turks buy Urals yesterday to cover Iranian [barrels]."
One Iranian fixture cited this week was Avin International's 153,000-dwt suezmax Episkopi (built 2001) to load on 27 or 28 May, but sources said this was part of a contract-of-affreightment pre-dating the US decision earlier this month.
Europe imports about 700,000 barrels per day, or 33% of Iranian crude shipments.
