BAGHDAD — Militants from the Islamic State group carried out a mass killing of hundreds of Iraqi soldiers captured when the extremists overran a military base north of Baghdad in June, a leading international watchdog said today.

The incident at Camp Speicher, an air base that previously served as a U.S. military facility, was one of the worst atrocities perpetrated by the Islamic State group as it seized large swaths of northern and western Iraq.

According to Human Rights Watch, new evidence indicates Islamic State fighters killed 560 to 770 men captured at Camp Speicher, near the city of Tikrit — a figure several times higher than what was initially reported.

“These are horrific and massive abuses, atrocities by the Islamic State, and on a scale that clearly rises to the crimes against humanity,” Fred Abrahams, special adviser to group, told journalists in the northern city of Irbil.

The al-Qaida-breakaway claimed in mid-June that it “executed” about 1,700 soldiers and military personnel from Camp Speicher.

The group also posted graphic photos that appeared to show its gunmen massacring scores of Iraqi soldiers after loading the captives onto flatbed trucks and then forcing them to lie face-down in a shallow ditch, their arms tied behind their backs.

Human Rights Watch said in late June that analysis of photos and satellite images showed that 160 to 190 men were killed in at least two locations between June 11 and 14.

The new Human Rights Watch report said the revised figure for the slain soldiers was based on analysis of new satellite imagery, militant videos and a survivor’s account that confirmed the existence of three more “mass execution sites.” The number of victims may well be even higher as more evidence emerges, it said.