Some Sunni clerics in the cities which are under the control of militants of the Islamic State of Iraq and the Levant (Isis) have been executed by the insurgents for not showing allegiance to the extremist outfit.

The militants are said to have executed about 12 leading clerics in the northern Iraqi city of Mosul.

Mosul, the second biggest Iraqi city, was the first city to be captured by the al-Qaida-linked militants, who are fighting against the Shiite-led Iraqi government in order to establish a separate state governed by hardline Sharia laws.

According to Al-Alam News, an imam in Mosul's Central Mosque was executed for refusing to join the insurgents in their mission.

Executions have also been reported from Tikrit in Salaheddin province.

The insurgents, who have taken to Twitter to brag about their executions, have tweeted footage of a security man being beheaded by them.

In one of the Twitter accounts, they have posted images of militants moving detained soldiers and security forces in trucks. The security men are then seen being forced to lie face down, with their hands tied, while a militant opens fire on their heads.

The group claims to have executed about 1,700 Shiite soldiers, but the numbers cannot be independently verified.

UN High Commissioner for Human Rights Navi Pillay expressed grave concern over the reports of "summary executions and extrajudicial killings".

UN human rights spokesperson Rupert Colville earlier warned that the international organization had received reports of several civilian casualties in the conflict.

Colville said: "The full extent of civilian casualties is not yet known, but reports received by the UN Mission in Iraq suggest that the number of people killed may run into the hundreds and the number of wounded is said to be approaching 1,000."