ISIS has executed three Imams in Mosul after they refused to recruit young people to the jihadists' cause.

Speaking from the city, Iraqi journalist Saaduddin al-Othman told ARA News that the three mosque leaders were beheaded publicly in central Mosul on Monday.

"The terrifying scene caused a popular outrage, especially that the victims were renowned figures and committed to true Islam; they have opposed ISIS atrocities under the name of Allah, that's why the terror group executed them," al-Othman said.

The three executed were identified as Kazim Abdulkarim, Bilal al-Sheikh Agha and Abdullah al-Hayalli.

ISIS issued a mandate last week that said all mosque leaders in Mosul must support jihad and encourage young people to join the militants.

Islamic State insists that it is following the 'true Islam', but its brutal regime has been denounced by the majority of Muslim leaders around the world.

Earlier this year, two Muslim clerics were among six people executed in Mosul for condemning the murder of Jordanian pilot Muath al-Kasasbeh.

Al-Kasaesbeh was captured by ISIS when his F-16 crashed over northeastern Syria last December. A video appearing to show him being burnt alive by militants was released in February this year.

Mosul was once considered the heartland of Iraq's Christian population, but was overrun by Islamic State jihadists on June 10 last year. It is now a key city in the militants' self-declared caliphate.