Friends of the Brisbane man in his 20s are worried he is planning to fight with Kurdish militia YPG against the Islamic State, according to reports from Seven and Nine News.

The man posted a photo to Facebook on Monday, wearing fatigues and posing with an assault rifle, with posts earlier this month showing him travelling though Jordan.

"No news is good news," he said.

"Miss everyone from home hope your (sic) all well #YPG #F***THEISLAMICSTATE."

Friends have flooded his page with messages of caution, urging him to rethink his apparent journey.

But many weren't convinced their friend was really planning to take up arms against the Islamic State.

"This better be a joke or I will ring your neck I swear to god," one said.

Another friend wrote: "You realise you can never return to Australia if this is all legit. What are you thinking."

Others seemed to be convinced the Brisbane man was serious, praying for his safety and telling him to stay safe.

"It's so believable though," one friend wrote.

"If it was anyone else it would have been a joke."

An Australian Federal Police spokesman and the Attorney-General's department both refused to comment on whether the Brisbane man was believed to be in a restricted zone or not.

But Nine News reported Kurdish fighters had confirmed they knew the man.

The man, reportedly 23 years old, appears to have been travelling since April, when he posted a photo from Amsterdam.

On May 5 he posted from Jordan's main airport, Queen Alia International Airport.

The Australian Government has consistently warned foreign fighters returning home would face jail.

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