Human Rights Watch says the Islamic State group massacred as many as 770 Iraqi soldiers who were captured when the militants overran an Iraqi military base in June.
The figure is much higher than what the New York-based rights group originally reported and is based on new evidence showing more execution sites.
Human Rights Watch said Wednesday the "barbarity" of Islamic State grossly offends the conscience.
Islamic State seized Camp Speicher near Tikrit in June, capturing hundreds of Iraqi soldiers and claiming they surrendered.
The militants posted graphic scenes of soldiers dressed in civilian clothes crouching as they were shot. Some of the bodies were pushed into ditches while others were thrown into the Tigris River.
The Iraqi army still lists many of the soldiers as missing. Some of their families stormed the Iraqi parliament Tuesday, demanding to know what happened to their loved ones.
President Barack Obama said Wednesday in Estonia that U.S. airstrikes have blunted the momentum of Islamic State militants in Iraq. He said the group needs to be degraded and destroyed.
"We know that if we are joined by the international community we can continue to shrink ISIL’s sphere of influence, its effectiveness, its financing, its military capabilities to the point where it is a manageable problem,' he said. "And the question is going to be making sure we’ve got the right strategy, but also making sure that we’ve got the international will to do it."
U.S. Defense Secretary Chuck Hagel told CNN television Wednesday that Islamic State cannot just be contained and reaffirmed Obama's comments.
Hagel said airstrikes alone will not do the job and that a functioning Iraqi government and a coalition are also needed.
The defense secretary said all options remain open, including military strikes on Islamic State targets in Syria.