A Muslim who claimed Islamic State had been a positive force in Mosul in Iraq is the main prosecution witness against an elderly Christian preacher who said in a sermon that Islam was "heathen" and "satanic".

Dr Raied Al-Wazzan, of the Belfast Islamic Centre, later withdrew the comments he made praising the work of Islamic State in Mosul, where only a few days ago four children were killed when another ancient church was blown up by the jihad terror group.

Pastor James McConnell, aged 78, has been charged under the 2003 Communications Act with "sending, or causing to be sent, by means of a public electronic communications network, a message or other matter that was grossly offensive." He faces up to six months in prison if convicted for the sermon preached last year and streamed online, in which he said: "Islam is heathen, Islam is Satanic, Islam is a doctrine spawned in Hell."

In legal documents seen by the Belfast Telegraph, Dr Al-Wazzan says in a statement to the Police Service of Northern Ireland that the pastor's comments were "terrible", "sweeping" and "offensive and disgusting". He says many Muslims in Northern Ireland are professionals, while by contrast, the pastor's congregation may include "impressionable, uneducated people".

Dr Al-Wazzan did not hear the sermon but became aware of it two days later when the Belfast Telegraph asked him for a comment.

The revelations that he is to be a prosecution witness come months after he defended Islamic State on the BBC.

Speaking about Mosul, Iraq's second city, Dr Al-Wazzan said on Radio Ulster in January: "Since the Islamic State took over, it has become the most peaceful city in the world." He added: "Yes, there are other things going wrong there... they are murdering people, I agree, but you can go from east to west of the city without fear."

After his comments led to widespread outrage, he withdrew them.

"I used the wrong words and I apologise for that," he said. He said he realised he had "offended many people" and added: "I condemn all kinds of violence."

McConnell told the Belfast Telegraph: "A man who praised the rule in Iraq of Isis murderers will be giving evidence against me - it's like a pantomime. I don't know how he will have the nerve to stand in a court and testify against me, and I don't know how the PPS has the nerve to think he is in a position morally to do that."

McConnell's lawyer Joe Rice said: "There are many bizarre features to this prosecution. We have now served our voluntary defence statement upon the court and the PPS. We expect the PPS to release additional and new information which may lead us to lodge an abuse of process application in the near future."

Northern Ireland's Public Prosecution Service had previously offered McConnell an "informed warning" for the offence, which he refused. A PPS spokesman said: "This case is now before the court and it is for the judge to decide on all evidential matters. It would be inappropriate for the PPS to make any further comment at this point."

The case expected to be heard in December.