RIYADH (AFP) - A Syrian professor at a Saudi Arabian university has disappeared and joined the ISIS (Islamic State in Iraq and Syria) group fighting US-backed forces in her homeland, a newspaper reported on Monday.

Al-Hayat quoted Ibrahim al-Khaldi, the spokesman for the University of Dammam, as saying Iman Mustafa al-Boga had resigned for unknown reasons.

It quoted from her Facebook page, which says she is "moving around in the north of Syria".

"I was Daesh even before Daesh existed," she says in another post, using the Arabic acronym for the group which has been accused of atrocities including crucifixions and beheadings. "And I have always known that there is no solution to the problems faced by Muslims but in this jihad."

The kingdom is seeking to deter youth from becoming extremists militants after Syria's conflict attracted hundreds of Saudis.

King Abdullah decreed in February jail terms of up to 20 years for citizens who travel to fight abroad, and the kingdom's top cleric has called ISIS and Al-Qaeda "enemy number one" of Islam.

Grand Mufti Sheikh Abdul Aziz al-Sheikh has urged Muslim young people not to be influenced by "calls for jihad... on perverted principles".

Since September Saudi Arabia and Qatar, along with their Gulf neighbours Bahrain and the United Arab Emirates, have taken part in or given support to US-led coalition air strikes against IS, which has declared a "caliphate" straddling Syria and Iraq.

Muslims around the world have denounced the militants.

"I left my dear university, my fancy car, my big house and large salary", to be free from "sinful laws of the tyrants that suffocate the ummah", Boga posted on October 14.

She later added that she had travelled without assistance from anyone.

"We do not know the reason behind her resignation," Al-Hayat quoted the university spokesman as saying.

The university "is keen to protect its students from ideas and methods that could be a danger to them, and could question their beliefs, traditions and customs", he added.

Al-Hayat reported that Boga, who was known to have extremist ideas, had spent many years at the campus teaching Islamic jurisprudence and economics.