A former Australian soldier has been killed fighting against Islamic State (IS) militants in north-eastern Syria.

Ashley Johnston, a former Army reservist originally from Queensland, was killed during an assault by Kurdish forces on an IS position near Tel Hamis, a tactically important area.

Mr Johnston served seven years in the Army Reserve as a rifleman and combat medic, before travelling to the Middle East last year to join the Kurdish YPG "self-protection" forces.

In a recent Facebook post he said he had served on peacekeeping operations, but that his time in the Australian Army was "uneventful".

A Defence spokesman said they were aware of the reports a former Army Reserve member had been killed, but would not comment further for privacy reasons.

The Kurdish forces had conducted two separate attacks on IS militants in the area in the day before Mr Jonhson's death on Monday evening (local time).

An American fighting with the Kurdish YPG militia, Jordan Matson, confirmed in a statement that Mr Johnston was killed by small arms fire during an IS attack.

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"Ashley was a good man who never complained and was always positive. He came to defend his country even when his country labelled him a criminal for doing so and before his country was willing to defend itself," Mr Matson said.

Mr Johnston served seven years in the Army Reserve as a rifleman and combat medic before travelling to the Middle East last year. ( Supplied: Facebook )

"I consider it an honour to have known and served with him."

The Reuters news agency reported that the YPG attack was backed by US air strikes, with Kurdish fighters supplying the coordinates of IS positions to the American-led coalition.

The operation killed 132 IS fighters, according to the Syrian Observatory for Human Rights, a monitor which is against the Syrian regime.

A group connected to the Kurdish forces called the Lions of Rojava described on Facebook how Mr Johnston, also known by the Kurdish name Bakok, was killed.

"On 23 Feb, at night and while fierce clashes were taking place between our units and the terrorists of [the Islamic State], comrade Bakok had fallen martyr after a heroic unpatrolled fight," it said.

The post stated that the Australian was killed in Gassan village, which the ABC understands was captured by YPG forces shortly after his death.

A friend of Mr Johnston's told the ABC that he was special person.

"I have never lost someone so close to my heart. Ashley was such and amazing, generous and loving person who I was so lucky to have met," she said.

"My heart breaks thinking of his family during this time and I will never forget the times we shared together."

An online tribute to Mr Johnston posted on the Lions of Rojava Facebook page described him as the first Westerner to be martyred fighting for the Kurds.

"Throughout his time in Kurdistan, he had a positive impact on my people's lives though his humility and kindness to everyone he met. He was taken from us in a heroic assault on ISIS positions in a small village near Shingal," the statement said.

"His squad of eight fighters where in a truck which had broken down and it was critical that they dislodge ISIS from their positions so they pushed on fearlessly with little regard for the own safety.

"They where (sic) massively outnumbered and outgunned but fearless in the face of this as they knew another ISIS death meant saving the lives of countless civilians. He was a fearless and exceptional soldier as well as a great man.

"Please keep his family and loved ones in your prayers and remember him and his heroic actions, which saved his comrades. He has given his life in the line of duty for his brothers in arms and for humanity in riding this world of the greatest evil of the 21st century. ISIS."

An American man, Robert Rose, who also fought with the Kurds against Islamic State, paid tribute to Mr Johnston on his Facebook page.

"RIP to my heval (friend) Ash," Mr Rose posted. "He was the first 1 when I got in country to teach the basics of my Ak 47 how to strip it assemble it and how to zero it, he was a great guy this really hurts I really feel bad for his family and friends."

Mr Johnston would appear to be the first Australian killed fighting against the Islamist group. ( Supplied: Facebook )

It is estimated about 20 Australians have been killed fighting for Islamic State in the Middle East, but Mr Johnston would appear to be the first killed fighting against the Islamist group.

Increasing numbers of Westerners are flocking to the region to fight with groups opposed to Islamic State.

Last month, the ABC revealed senior Northern Territory Labor figure Matthew Gardiner had left Australia allegedly to join up with the Kurds fighting against IS, while Melbourne man Khamis Gewargis Khamis travelled to northern Iraq to fight in an Assyrian Christian militia against IS.

The Department of Foreign Affairs and Trade said in the statement that it was aware of the reports of Mr Johnston's death.

"The Australian Government's capacity to confirm reports of deaths in either Syria or Iraq is extremely limited," the department said in a statement.

"Due to the extremely dangerous security situation, consular assistance is no longer available within Syria. Australians who become involved in overseas conflicts are putting their own lives in mortal danger.

"Any Australians fighting with non-state militia in Syria or Iraq should end their involvement in the conflict now and leave the conflict zone. Australians are strongly advised not to travel to Syria or Iraq; any Australians in either country should leave immediately."