An Australian preacher who has emerged as one of the highest-profile supporters of the Islamic State of Iraq and Syria (ISIS) has claimed to be travelling to Syria.

Melbourne-born Musa Cerantonio has reportedly been living in the Philippines but broke his nearly three-week silence on Twitter overnight to say he was headed to Syria.

"Insha'Allah I will be arriving in Ash-Sham very shortly, keep us in your du'a, getting ready to travel," he wrote, referring to the Arabic name for Syria.

Mr Cerantonio also tweeted support for Abu Bakr Al-Baghdadi, the head of ISIS, who this week proclaimed an Islamic state encompassing areas of Iraq and Syria.

"Al-Hamdulillah the establishment of the Khilafah in this blessed month is a glad tiding for all the Muslims and brings great joy to us," he wrote.

His message generated an enthusiastic response from his Twitter followers, many of whom wished him a safe journey.

Mr Cerantonio was born into an Italian family but converted to Islam at the age of 17 and has emerged as one of the most popular and influential online preachers supporting the jihad in Iraq and Syria.

He has been likened to Al Qaeda's most prolific online preacher, Anwar Al Awlaki, who was linked to attempted attacks in the US. Al Awlaki was killed in the 2011 drone strike in Yemen.

"He appears to have some of the same appeal as the hip, young sheikh who speaks to the guys in the street, who's not fuddy-duddy uncle or fatherly figure, but a brother you can relate to," Greg Barton of Monash University's Global Terrorism Research Centre told ABC's 7.30 program last month.

"So in that sense, he's like Anwar Al Awlaki. He's speaking very directly and very persuasively."

A recent report by the International Centre for the Study of Radicalisation described Mr Cerantonio as an important "spiritual authority" for foreign fighters in Iraq and Syria, but said there was no evidence he was facilitating the flow of jihadists to those countries.

It said his Facebook page is the third most "liked" by jihadists and that one in four foreign fighters follows his Twitter account.

"Cerantonio typically employs highly inflammatory language, for example, calling the State Department 'pussy Yankee scum', and claiming that 'the USA and its slaves like you are the greatest criminals on Earth'," the report said.

"In one instance, he posted a modified image of the US State Department seal which read 'US Department of Rape'."

Meanwhile Attorney-General George Brandis has met with Islamic leaders at Parliament House in Canberra to discuss the problem of so-called home-grown terrorism.

After the talks, Islamic leaders said they will do what they can to stop young Australians heading overseas to fight alongside extremists.

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