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ISIL, radical Islamists who want to re-create a mediaeval-style caliphate straddling Iraq and Syria, has stormed largely unopposed across much of northern Iraq, taking cities including Mosul and Tikrit, seizing border posts with Syria and advancing to within some 100km of the capital Baghdad.
New York-based Human Rights Watch said that between 160 and 190 men were killed in at least two locations in and around Tikrit - the hometown of late Iraqi dictator Saddam Hussein - between June 11 and 14.
It said that the death toll could be much higher but the difficulty of locating bodies and getting to the area had prevented a full investigation.
Pictures posted on HRW's website showed a row of men lined up face-down in trenches being shot by gunmen.
"The photos and satellite images from Tikrit provide strong evidence of a horrible war crime that needs further investigation," Human Rights Watch emergencies director Peter Bouckaert said in a statement.
"They and other abusive forces should know that the eyes of Iraqis and the world are watching."
It was not immediately possible to get comment from ISIL.
AT LEAST 1000 KILLED THIS MONTH -UN
The United Nations said on Tuesday that at least 1,000 people, mainly civilians, had been killed and roughly the same number injured in fighting and other violence in Iraq in June as ISIL swept through the north. Victims included a number of confirmed summary executions committed by ISIL as well as prisoners killed by retreating Iraqi forces.
Human Rights Watch said ISIL posted about 60 photographs on its Twitter feed on June 14, showing fighters loading captives in civilian clothes onto trucks and then forcing them to lie in three shallow trenches.
HRW counted the bodies visible in the available photographs, and estimated that ISIL killed between 90 and 110 men in one trench and between 35 and 40 men in the second.
A further photograph shows a large trench with between 35 and 40 prisoners shot but Human Rights Watch said it had not been able to pinpoint the site.
A U.N. human rights spokesman said on Tuesday that ISIL had broadcast dozens of videos showing cruel treatment, beheadings and shootings of captured soldiers, police officers and people apparently targeted because of their religion or ethnicity, including Shi'ites and minorities such as Christians.
Northern units of Iraq's million-strong army, trained and equipped by the United States, largely evaporated after Sunni Islamist fighters led by the ISIL launched their assault.
In Tikrit on Friday, Iraqi army helicopters fired on a university campus in an effort to dislodge ISIL fighters, a day after launching an airborne assault on the city.