Five years after Australia's combat operations officially ended in Afghanistan, an elite Australian soldier remains on the front line in the war-torn country, fighting Islamic State (IS) militants.

Key points: An Australian special forces soldier known as "Mark" is serving in Afghanistan with US green berets

An Australian special forces soldier known as "Mark" is serving in Afghanistan with US green berets They are based at a remote outpost in the Nangarhar Province

They are based at a remote outpost in the Nangarhar Province The Special Forces soldiers are targeting ISIS-K, a local offshoot of Islamic State

The Special Forces soldier known as "Mark" is attached to a group of elite American "Green Berets", who have been operating in eastern Afghanistan's isolated Mohmand Valley, near the Pakistan border.

His dangerous "advise and assist" mission in the remote Nangarhar province has been documented by journalist Marty Skovlund Jr, who also once served as an American Special Operations soldier.

A US Special Forces soldier fires a mortar round ( Supplied: Coffee, or Die Magazine )

Earlier this year, Skovlund spent several days embedded at Combat Outpost (COP) Blackfish, as US Special Forces helped Afghan Commandos fight ISIS-K, a local offshoot of the brutal IS terrorist organisation which rose to prominence in Iraq and Afghanistan.

In an article just published in the American "Coffee, or Die" Magazine, Skovlund revealed the presence of the sole Australian Special Forces soldier who is serving with the Americans while on an exchange program with the US military.

US Special Forces Combat Outpost Blackfish. ( Supplied: Coffee, or Die Magazine )

At present Australia has around 300 personnel based in Afghanistan as part of Operation Highroad, which supports a NATO-led train, advise and assist mission, but very little is known about the Special Forces soldier "Mark".

Australia's Defence Department has declined to comment on whether any other Australian military personnel are involved in combat operations against IS in Afghanistan.

A US special forces team sergeant returns fire during a fight in Afghanistan ( Supplied: Coffee, or die Magazine )

ISIS-K a small but 'highly dangerous' group

Skovlund said ISIS-K were still holding on in Afghanistan "mostly because they can, mostly because it's not nearly as easy to target them, as it was in Iraq and Syria, so they're holding on to these certain valleys".

The reporter said Afghan soldiers told him the local population was far more fearful of the ISIS-K terrorists, than they were of the Taliban.

A US solider calls in airstrikes over Bagh Dara Valley. ( Supplied: Coffee, or Die Magazine )

"They're drug addicts, or mentally unstable folks … whereas the Taliban is much more your regular Afghan that has a different take on what the direction of the country should be."

"ISIS came down out of the mountains into the village for what I would call this day raid… and literally just took a random guy out of the crowd and beheaded him in front of the entire village."

Amin Saikal from the Australian National University said ISIS-K was a relatively small, but highly dangerous, terrorist group.

"ISIS-Khorasan, which is basically a branch of ISIS established in Afghanistan over the old territory comprised of parts of Afghanistan, Iran, Turkmenistan and Uzbekistan … is estimated to be around 1000 to 2000 right across the country," Professor Saikal said.

"They are really made up of disgruntled Pakistani Taliban and Afghan Taliban who've simply broken away from the mainstream in the two countries.