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 Wednesday.

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 between 560 and 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.

After the incident, the soldiers were listed as missing, prompting their families to stage demonstrations in Baghdad in an effort to pressure authorities for word on their sons' fate. On Tuesday, dozens of angry family members stormed into the parliament in Baghdad's fortified Green Zone after scuffling with security guards. They forced the speaker to call a session Wednesday on the missing soldiers.

Human Rights Watch said in late June that analysis of photos and satellite images showed that between 160 and 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.

"The barbarity of the Islamic State violates the law and grossly offends the conscience," said Peter Bouckaert, emergencies director at Human Rights Watch.

At Wednesday's parliament session, the soldiers' families accused authorities of "selling our sons" by ordering many of the soldiers to abandon their posts and leave Camp Speicher in civilian clothes.

Once outside the base, hundreds were captured, said Mohammed al-Assi, a representative for the soldiers' families.

However, acting Defense Minister Saadoun al-Dulaimi denied any orders to abandon Camp Speicher.

During the parliament session, survivor Thaer Abdul-Karim told lawmakers that a military commander ordered the soldiers in Speicher on June 12 to leave the camp and  hand over their weapons .

Abdul-Karim said the commander told the troops that there were military trucks waiting for them at a nearby highway to take them to a base near Baghdad. Instead, the soldiers, in civilian clothes, were taken by gunmen who were waiting for them on the highway.

The gunmen later ordered batches of prisoners to go out and started to shoot them.

"We panicked after seeing our colleagues being shot dead," Abdul-Karim said. "There was a state of chaos and some started to run away and I managed to escape from the place."

Also Wednesday, the U.N. envoy in Iraq called for a public and independent investigation by Iraqi authorities into the fate of the missing soldiers and the recovery of the remains of those killed. Outgoing Prime Minister Nouri al-Maliki said that a number of "perpetrators" of Camp Speicher atrocities have been arrested or killed and that "security forces were pursuing" others. Al-Maliki did not elaborate.

The onslaught by the Islamic State group has stunned Iraq's security forces and the military, which melted away as the extremists advanced and captured key cities and towns. The militants also targeted Iraq's indigenous religious minorities, including Christians and followers of the ancient Yazidi faith, forcing tens of thousands from their homes.

Since then, the Islamic State group has carved out a self-styled caliphate in the large area straddling the Iraqi-Syrian border that it now controls.

In early August, the United States launched airstrikes on the militant group in Iraq, in an effort to help Iraqi forces fight back against the growing militant threat.

On Tuesday, the Islamic State group released a video showing the beheading of American-Israeli journalist, Steven Sotloff, and warned President Barack Obama that continuing airstrikes against the group in Iraq will be met with the killing of more Western captives. The footage was posted two weeks after the release of a video showing the killing of U.S. journalist James Foley.