Iraqi insurgents executed at least 160 captives earlier this month in the northern city of Tikrit, Human Rights Watch has said, citing an analysis of satellite imagery and grisly photographs released by the militants.

The US-based group said militants from the Islamic State of Iraq and the Levant (Isis) killed between 160 and 190 men in two locations in Tikrit between 11 and 14 June.

"The number of victims may well be much higher, but the difficulty of locating bodies and accessing the area has prevented a full investigation," it said.

After overrunning large swaths of northern Iraq and capturing the cities of Mosul and Tikrit earlier this month, the Islamic extremist group posted graphic photos on a militant website that appeared to show fighters loading dozens of captured soldiers on to flatbed trucks before forcing them to lie in a shallow ditch with their hands tied behind their backs. A final set of photos shows what appear to be dead bodies.

"The photos and satellite images from Tikrit provide strong evidence of a horrible war crime that needs further investigation," Peter Bouckaert, emergencies director at Human Rights Watch, said in a statement. The militants "apparently executed at the very least 160 people in Tikrit".

Image posted on a militant website appears to show Isis militants with Iraqi prisoners in Tikrit. Photograph: AP

Chief Iraqi military spokesman Lt Gen Qassim al-Moussawi confirmed the photos' authenticity after they surfaced on 15 June and said he was aware of cases of mass murder of captured Iraqi soldiers in areas held by Isis.

He said at the time that an examination of the images by military experts showed that about 170 soldiers were shot to death by the militants after their capture.

Captions on the photos showing the soldiers after they were shot say "hundreds have been liquidated", but the total could not immediately be verified.

The massacre appeared to be aimed at instilling fear in Iraq's demoralised armed forces – which melted away as militants seized much of the north in a matter of days – as well as the country's Shia majority, whom Isis views as apostates.

The massacre appeared to be aimed at instilling fear in Iraq's demoralised armed forces. Photograph: AP

Human Rights Watch said its analysis of the photos showed two trenches in the same location, and that using satellite imagery from 2013 and publicly available photos taken earlier it was able to pinpoint the execution site in a field near a former palace of late dictator Saddam Hussein next to the Tigris river.

It said satellite imagery of the site from 16 June did not reveal bodies but showed indications of vehicles and earth movement consistent with the two shallow trenches visible in the photos.