Islamic State terrorists have hacked the website of a Catholic church, posting a beheading video and a message warning that the jihad will not cease until Rome is conquered.

The jihadists are believed to have picked on Our Lady of Mercy Syriac Catholic Church in Sydney, Australia because many of its members are refugees who have fled the conflicts in Syria and Iraq. The church had recently launched an appeal for help for the refugees. The video, now taken down, showed beheadings and terrorised bodies.

The written text, tagged #thefailedcrusade, said: "Now our fighting has come! We don't negotiate except with cannon, we don't have dialogues except with guns, we will not talk except strength. And we will not stop the fighting until we make Athan and pray in Rome by Allah's will in a conquest."

Many of the worshippers in the church have lost everything. 

The parish wrote on Facebook that the terrorists were seemingly not satisfied with killing and displacing Christians, but now wanted to deprive them of the help and assistance they were receiving in their new homes. 

The hackers also inserted a link to a website which described the Paris attacks as a "blessed onslaught".

"Please pray for the church and to the success of this organisation and pray for all our displaced people," the parish wrote. The church has been in contact with Australian Security Intelligence.