The Islamic State group Tuesday defended its destruction of religious sites in the Iraqi city of Mosul on the grounds that the use of mosques built on graves amounted to idolatry.

"The demolition of structures erected above graves is a matter of great religious clarity," the jihadist group said in a statement posted on one of its main websites.

"Our pious predecessors have done so... There is no debate on the legitimacy of demolishing or removing those graves and shrines," the Islamic State (IS) said.

It cited the demolition by Mohammed bin Abdel Wahhab -- founder of the puritanical Wahhabist brand of Islam followed by jihadists -- of a dome erected above the tomb of Zaid ibn al-Khattab.

Khattab's reputedly heroic death on the battlefield earned him a posthumous following which Abdel Wahhab argued was tantamount to polytheism.

IS, which announced the restoration of the caliphate last month by declaring its sovereignty over land it has seized in Syria and Iraq, has levelled several of Mosul's most prominent religious landmarks.

They include the Nabi Yunus shrine (Jonah's tomb) and a shrine to Prophet Seth -- considered in Islam, Judaism and Christianity to be Adam and Eve's third son.

Mosul's new jihadist rulers also threatened to blow up the so-called "hunchback" (Hadba), a leaning minaret built in the 12th century and one of Iraq's most recognisable landmarks.

IS insisted that all schools of Islamic law "agreed that using graves as mosques was un-Islamic" since it amounted to idolatry.

However, many Islamic scholars, including hardliners, have strongly disagreed.

Harith al-Dhari, a Sunni cleric who lives in exile in Jordan and chairs Iraq's Committee of Muslim Scholars, has condemned the demolitions.

"The committee would like to underline the huge loss for the people of Mosul, who saw these blessed mosques as landmarks of the city, a part of its culture and history," said Dhari, who was long seen as sympathetic to Islamists in Iraq.