Iran has officially charged three detained Australian citizens with spying, judiciary spokesman Gholamhossein Esmaili said on Tuesday in a video message.

This was the first official confirmation that the three have been jailed, after Canberra said last week that the Australians had been detained, naming the trio as backpacking couple Jolie King and Mark Firkin (above) and Melbourne University lecturer Kylie Moore-Gilbert.

"The news is correct," Esmaili said.  "One case is of two people taking photos of military sites and our forbidden areas," he said, "the pair had been identified at the time and images were found on a drone they were using."

The third case involved an individual accused of "spying for another country," he added.

Suspicion began to arise about the whereabouts of King and Firkin after they abruptly stopped blogging about their travels 10 weeks ago, following posts from Pakistan and Kyrgyzstan. The family of Moore-Gilbert told the media that she has been detained for a "number of months."

Australian Foreign Minister Marise Payne said there was "no reason" to believe the arrests were politically motivated. However, the news of the detentions came shortly after Prime Minister Scott Morrison announced a "modest" Australian contribution to a US-led coalition trying to protect oil tankers in the Strait of Hormuz following a series of attacks.

Canberra has said that consular assistance was being given to the three detainees.

es/msh (AFP, dpa)