Saudi Arabia's highest religious authority has condemned the armed groups Islamic State and al-Qaeda as apostates and labelled them the "number one enemy of Islam".

Live Box 2012108933861442

The country's grand mufti, Abdulaziz al-Sheikh, said on Tuesday that terrorism, which he accuses the groups of committing on a systematic scale, had no place in Islam's ideology.

"Extremist and militant ideas and terrorism which spread decay on Earth, destroying human civilisation, are not in any way part of Islam, but are enemy number one of Islam, and Muslims are their first victims," he said in a statement carried by the official Saudi Press Agency.

The Saudi government has been a main supporter of rebels battling Syrian president Bashar al-Assad, but has funnelled arms and money away from Islamic State and al-Qaeda, and towards other opposition groups in the country.

The Islamic State, which according to recent reports has tens of thousands of fighters, has seized control over large parts of Syria and Iraq, where they have been accused of committing mass atrocities against civilians.

Al-Sheikh's statements came a almost a year after he urged Saudi citizens to not travel to Syria to take part in the more than three-year-long conflict.

The grand mufti, appointed by the Saudi king, also warned preachers against encouraging young men to fight in Syria during their sermons, after delivering what the paper said was a lecture on "deviation among the youth" at a mosque.

Source: Agencies