The sailor, whose identity was not released, was reported missing on Wednesday "after a reported man overboard incident," the Bahrain-based Fifth Fleet said in a statement.

Ships from Spain and Pakistan together with the US are looking for the sailor, it said.

The incident came as Iran on Thursday said it had detained a "foreign tanker" and its 12-person crew for allegedly smuggling fuel amid a tense standoff between Tehran and the US and its allies in the Gulf.

Washington is seeking to form an international coalition to escort merchant ships in the region, and US Central Command chief Kenneth McKenzie pledged to work "aggressively" to ensure freedom of navigation in the highly sensitive waters through which much of the world's crude oil travels.