Two US Navy boats were taken into Iranian custody but Iran has told the United States that the crews will be returned "promptly".

A US official said: "Earlier today, we lost contact with two small US naval craft en route from Kuwait to Bahrain.

"We subsequently have been in communication with Iranian authorities, who have informed us of the safety and well-being of our personnel.

"We have received assurances the sailors will promptly be allowed to continue their journey."

Iranian Foreign Minister Mohammad Javad Zarif has assured US Secretary of State John Kerry that the sailors would be allowed to continue their journey, another US official said.

The two craft were manned by a total of ten sailors, defence officials said.

Officials say it is unlikely that they will be released overnight.

The US craft patrolling the Gulf were somehow disabled and drifted into Iranian territorial waters at which point they were detailed by Iranian Revolutionary Guards and taken to Farsi Island in the middle of the Gulf, Fox News reported.