There will be "serious consequences" if Iran does not release a British-flagged tanker seized in the Gulf, Foreign Secretary Jeremy Hunt has said.
The Stena Impero's owners say they have been unable to contact their vessel, which had 23 personnel on board and was "heading north towards Iran".
It was surrounded by four vessels and a helicopter, Mr Hunt said.
A second British-owned Liberian-flagged tanker was boarded by armed guards but is now free to continue its journey.
The Mesdar's Glasgow-based operator Norbulk Shipping UK said communication had been re-established with the vessel after it was boarded by armed guards at around 17:30 BST on Friday and its crew was "safe and well".
There were 25 personnel on board the Mesdar when it was surrounded by 10 speedboats, Mr Hunt said.