The death of the man from south-west Sydney follows still unconfirmed reports that Islamic State kingpin, and Australian, Mohammad Ali Baryalei and another IS fighter by the name of Abu Noor al-Kurdi have also been killed.

The three deaths bring the suspected death toll of Australians in Syria and Iraq to 13 and comes as US President Barack Obama authorised Defence Secretary Chuck Hagel to send up to 1500 additional US troops to Iraq. That move doubles the force the US has built up since June to fight the IS militants controlling much of Iraq and Syria.

Sydney community sources have told The Sun-Herald that the man was from a well-known family and understood to have been married with children. The circumstances of his death are unknown.

A spokesman for the Department of Foreign Affairs and Trade said the Australian government is attempting to verify the reports of the men's deaths.

"Due to the extremely dangerous security situation consular assistance is no longer available within Syria or Iraq. The government's capacity to follow up reports of this kind is therefore extremely limited. Australians are strongly advised not to travel to Syria and Iraq."

News of the killing of IS fighters has in the past been followed by a flurry of social media, however there has been little online chatter about the three men.

The increasing role of foreign fighters has led to the cancellation of dozens of Australian passports by Australian security agencies, the introduction of tough new laws and also to police raids in Sydney and Brisbane in September.

Among those who have travelled overseas to join the fighting are two teenagers from Bankstown, 17-year-old Abdullah Elmir and his 16-year-old friend Feiz.

Feiz has since returned to Sydney and but Abdullah has joined IS in Iraq and become their poster boy appearing in a number of propaganda videos which appeared on social media last month including one threatening the Prime Minister Tony Abbott.

Baryalei, a former Kings Cross bouncer and part-time actor, had been working for IS on the Turkey-Syria border, receiving new fighters from Australia and facilitating their passage into Syria to join terrorist groups.

Baryalei, 33, who grew up in Quakers Hill, is understood to have left for the Middle East in April 2013, telling his friends he was going to study sharia in Turkey.

The Australian Federal Police had issued a warrant for his arrest earlier this year due to terrorism-related offences.

It is believed he had risen to become a senior leader in Islamic State (formerly known as ISIS) and had facilitated the movement of up to half of the 60 Australians believed to be fighting  in Syria and Iraq.

Some of the Australian fighters he allegedly recruited include Khaled Sharrouf and Mohamed Elomar. He is also believed to have recruited Yusuf Ali, Amira Karroum and Caner Temel who have all since been killed.

News of Baryalei's death late last month was followed closely by reports that Abu Noor Al-Kurdi had been killed with him in the Syrian town of Kobane. It is understood he was also from south-west Sydney. A memorial service was held last week in Auburn, but community members contacted about it declined to comment.

The additional US troops will expand a military advisory mission in Iraq and will establish a new effort to train Iraqi forces.

Asked whether Australia had been asked to commit more troops to the fight against the Islamic State, Mr Abbott said on Saturday while the government continued to talk with partners and allies about how best to defeat the militants, Australia had already made "a strong commitment".

"Our commitment is clear: it's up to eight Super Hornet strike aircraft, it's a refueller, it's an airborne early warning and control aircraft, it's up to 200 Special Forces," Mr Abbott told reporters in Melbourne.

Speaking in Beijing, Foreign Minister Julie Bishop said Australia's contribution to the fight against Islamic State was "crucial, but it's also proportionate".

Asked about the US decision to send an additional 1500 troops to Iraq, Ms Bishop said "I don't think that anybody was under any illusion that this would be easy".

"ISIL is well-funded, well-resourced. Apparently 16,000 fighters or more from 80 different countries have joined ISIL's forces in Syria and Iraq so it will take time, it will take the effort of a number of countries," she said.