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The United Nations reports that Islamic State of Iraq and the Levant (Isis) fighters carried out summary executions of civilians and members of the Iraqi army in the northern city of Mosul.
A UN spokesman says the number of killings could run into the hundreds. Unconfirmed reports say 1,700 Shia soldiers have been executed.
"We've also had reports suggesting that the government forces have also committed excesses, in particular the shelling of civilian areas on 6 and 8 June," said UN human rights spokesman Rupert Colville.
"There are claims that up to 30 civilians may have been killed."
A summary execution is a killing committed after a person is accused of a crime and slain without a full and fair trial. The methods of execution committed by the militants remain unclear. Unconfirmed reports have claimed beheadings and crucifixations are being used.
The group have continued their advance towards Baghdad after capturing Mosul and former Iraq dictator Saddam Hussein's hometown of Tikrit. The militants have now moved into two towns, Saadiyah and Jalawla, in the eastern province of Diyala.
US President Barack Obama has said that he is looking at "all options" to help Iraqi prime minister Nouri al-Maliki thwart the insurgency.
"I don't rule out anything because we do have a stake in making sure that these jihadists are not getting a permanent foothold in either Iraq or Syria," Obama said.