Underscoring the deep concern the Abbott government has expressed in recent weeks about Australians involved with extremist groups, two prominent jihadists fighting with the al-Qaeda splinter group the Islamic State took to Twitter to issue fresh threats.

Mohamed Elomar, a former boxer from Sydney who has been implicated in executions of unarmed Iraqis, posted on the social media site a distressing picture of what appears to be a captured Iraqi soldier who has been beaten and is about to be executed.

The accompanying Twitter message states: ''Look at the end of these Iraqi maliki dogs bunch of girls can't wait to see a Australian soldier cyring [sic] saying bakia.''

''Bakia'' means ''he cries'' in Arabic and ''maliki'' refers to the Shiite-led Iraqi government of Nouri al-Maliki, the fervent enemy of the Sunni extremists who have swept through northern Iraq in recent weeks.

Convicted Australian terrorist Khaled Sharrouf, who appeared in Facebook images recently brandishing a gun over executed Iraqis, replied on Twitter: ''Allah … says 'kill them where ever you find them' … terrorise the enemies of Allah.''

Both Elomar and Sharrouf are known to be in Iraq with the ultra-violent group the Islamic State (IS), formerly the Islamic State in Iraq and the Levant. The group changed its name recently after declaring a caliphate, or global Muslim state, in the territory it has captured spanning Syria and Iraq.

Sharrouf was convicted over the 2005 Pendennis terrorism plot and jailed for nearly four years, before slipping out of Australia in December on his brother's passport to join the fighting in Syria. He and Elomar are believed to have crossed the border in recent weeks with IS fighters as the group surged across northern Iraq seizing major towns and cities.

A small force of Australian troops, including elite members of the SAS, travelled to Baghdad just over a fortnight ago to help secure the Australian embassy, though it is regarded as unlikely that IS fighters could reach the Iraqi capital. The Abbott government has ruled out sending troops into combat in the country.

Meanwhile Melbourne-born extremist preacher Musa Cerantonio declared on Twitter that he had reached the ISIL-controlled territory in Syria and Iraq, reflecting the potentially dangerous gravitational effect of the group's caliphate declaration.

''Al-Hamdulillah [Thanks to God] I have arrived in the land of Khilafah [caliphate] in Ash-Sham [the Levant]!'' he wrote on Twitter. Sharrouf replied: ''Alhumdulihe [Thanks to God] on your arrival see u soon akhi [brother].''

The case of Cerantonio is particularly troubling because of his status as a top cheerleader for IS with a massive online following. A recent report by the International Centre for the Study of Radicalisation and Political Violence at King's College, London, described him as one of the three most influential preachers online.

He appears to have slipped out of the Philippines where he was in hiding in recent months. His arrival in the Middle East is a blow to Australian authorities who were reportedly moving to extradite him from the Philippines.

Andrew Zammit, a researcher with Monash University's Global Terrorism Research Centre, said the fact there had been six plots in Europe by returnees from Syria reflected the significant threat level of Westerners fighting in the region.

''Most of the fighters will likely confine their violence to the region, but the prospect that some will return and carry out violence at home is real,'' he said. He said Cerantonio had previously hedged his bets between key jihadist groups but now appeared to have ''gone all in'' with his support for IS and its new caliphate.

IS is in a power struggle with the old al-Qaeda of Osama bin Laden as well as with another al-Qaeda splinter group, Jabhat al-Nusra.

Some experts have said this increases the danger of a major attack on the West.

With Rachel Olding