Islamic State Iraq execution
Isis has released photos depicting the execution of 13 men in Iraq Twitter

As a self-professed Islamic State (Isis) sympathiser was holding 17 people hostage in Australia, the extremist group extended its list of atrocities in Syria and Iraq, with a mass execution and a double beheading.

Photos of the two separate killings in Iraq's central Salah al-Din province and in Syria's Homs region were posted online by jihadist militants.

Isis Salah al-Din provincial division released three pictures showing the public execution of 13 men said to be members of a rival Sunni Muslim militia.

In the first photo, the men are seen in line on their knees on a roadside wearing prisoner-like orange jumpsuits, backed by a line of black-clad Isis militants with their faces covered.

The other photos depict the prisoners being shot dead in front of a crowd of onlookers.

According to Site Intelligence Group, the victims were accused of being members of the "Knights of Knowledge" Awakening group, one of the Sunni Muslim militias that supported US troops against Isis Iraqi-based predecessor, al-Qaeda in Iraq, in 2006.

The second set of photos released by Isis Homs provincial division shows the beheading of two men, one accused to be a spy and the second of being a pro-government fighter.

The two are slayed in an unpaved square of a village in the countryside near the city of Homs. Once again the execution is carried out in front of a few dozen onlookers, including children.

A day before the pictures were posted online, activists with the Syrian Observatory for Human Rights (SOHR) reported that several men were beheaded by Isis Islamic Police on charges of "insulting almighty Allah" in eastern countryside of Homs.

SOHR was initially informed that the number of victims was four.

Isis is known for its brutality and has held public and mass executions in the recent past. In numerous cases, including those involving Western hostages James Foley, Steven Sotloff, David Haines, Alan Henning and Peter Kassig, footage or pictures of the gruesome killings have been posted online for propaganda purposes.

The practice has been condemned even by senior commanders with rival jihadi group al-Qaeda.

The latest killings were revealed as the world was watching the dramatic hostage crisis at a Sydney café.

A gunman, identified as Iranian native Man Haron Monis, 50, burst into the Lindt Cafe in Martin Place and took 17 people inside as hostages.

Monis, who claimed to be acting on Isis' behalf, was shot dead as police stormed the premises after a 16-hour standoff. Two hostages were also killed.