Satellite imagery strongly suggests the extremist Sunni group the Islamic State of Iraq and Syria (ISIS) has conducted mass executions in Iraq, lobby group Human Rights Watch says.

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ISIS, radical Islamists who want to re-create a medieval-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 100 kilometres of the capital Baghdad.

In mid June, ISIS posted pictures on Twitter that they claimed showed the massacre of 1,700 Iraqi soldiers after the fall of the city of Tikrit.

However, there has been no independent verification.

New York-based Human Rights Watch (HRW) 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.

"The photos and satellite images from Tikrit provide strong evidence of a horrible war crime that needs further investigation," HRW 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."

Caroline Anning, from Save the Children in Erbil, says the ISIS offensive in recent weeks has prompted thousands of people to flee their homes.

"What I've seen is thousands of people fleeing every day and fleeing for their lives and what we know is that it's an extremely violent and chaotic situation in the north of Iraq," she told ABC News 24.

"Certainly atrocities are taking place and, you know, people are terrified and with good reason and that's why they're dropping their things and they're leaving their lives behind."

It was not immediately possible to get comment from ISIS.

1,000 killed in fighting: 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 ISIS swept through the north.

Victims included a number of confirmed summary executions committed by ISIS as well as prisoners killed by retreating Iraqi forces.

HRW counted the bodies visible in the available ISIS photographs and estimated that ISIS 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 UN human rights spokesman said on Tuesday that ISIS 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 Shiites and minorities such as Christians.

Northern units of Iraq's million-strong army, trained and equipped by the US, largely evaporated after Sunni Islamist fighters led by the ISIS launched their assault.

In Tikrit on Friday, Iraqi army helicopters fired on a university campus in an effort to dislodge ISIS fighters, a day after launching an airborne assault on the city.

The United States, which has deployed 180 troops to Iraq as military advisers, confirmed it was flying "a few" armed drones over Baghdad to defend American troops and diplomats if necessary.

"For the last 24 to 48 hours, we've started that," the official, who spoke on condition of anonymity, told AFP.

Influential cleric calls for new prime minister

Meanwhile, Iraq's most influential Shiite cleric, Grand Ayatollah Ali al-Sistani, has called on the country's leaders to choose a prime minister within the next four days.

The dramatic intervention could hasten the end of prime minister Nouri al-Maliki's eight-year rule.

Grand Ayatollah Sistani's message was delivered after a meeting of Shiite factions, including Mr Maliki's State of Law coalition, failed to agree on a consensus candidate for prime minister.

The embattled Mr Maliki accused his political foes of trying to prevent parliament from meeting on time and stirring up violence to interfere with the political process.

"They worked to postpone the elections ... and now they are working to postpone the first session of the council of representatives," he said during a televised meeting with commanders.

"If they are not able to pressure us to postpone, they will go for inciting security incidents in Baghdad."

ABC/Reuters