US sailors in Iran made 'mistake in navigation', White House confirms

The 10 sailors, who the Pentagon says were not on a secret mission, have now been returned to the US fleet

The detention of American Navy sailors by the Iranian Revolutionary Guard in the Persian Gulf on Tuesday after their boats drifted into Iranian territorial waters (AP)

Defence Secretary Ash Carter has said a mistake in navigation caused two US navy boats to enter Iranian waters earlier this week.

"I think that this much is clear, there was a navigational error of some kind," he told a press conference on Thursday.

"All the contributing factors to that we don't know yet, and we're still talking to those folks, and we'll find out more ... but they were clearly out of the position that they intended to be in."

He also said the sailors were not on a covert mission and were simply making their way from Kuwait to Bahrain.

Defence officials said the main cause behind the Riverine boats entering Iranian waters was the navigational slip, apparently caused by human error rather than equipment malfunction.

Along with the navigational error, one of the boats experienced engine trouble, preventing them from evading the Iranians once the crews realised they were inside Iran's territorial waters.

A US Navy Riverine Command Boat (RCB) 805 in the Persian Gulf (file pic) (AP)

The sailors, nine men and one woman, were captured and held overnight at an Iranian base on Farsi Island.

Iranian state media broadcast footage of the sailors being detained and apologising.

Less than a day after being detained, all 10 sailors were returned to the American fleet. The US Navy is now conducting a full debrief with the crew.