An international rights group says Islamic State militants carried out a mass killing of hundreds of Iraqi soldiers captured when the extremists overran a military base north of Baghdad in June.

Human Rights Watch said Wednesday that new evidence indicates the Islamic State group killed between 560 and 770 men captured at Camp Speicher, near the city of Tikrit.

The New York-based watchdog says the number of slain Iraqi soldiers is several times higher than previously reported. Earlier, Human Rights Watch said between 160 and 190 men were killed.

Human Rights Watch says the new number is based on analysis of new satellite imagery, militant videos and a survivor's account.

In June, the Islamic State group claimed it had "executed" about 1,700 soldiers and military personnel captured from Camp Speicher.