Iraqi forensic teams in the newly recaptured city of Tikrit have started exhuming bodies from mass graves believed to contain some of the hundreds of soldiers killed by Islamic State militants last year, a government spokesman said Tuesday.

Kamil Amin, from Iraq's Human Rights Ministry, said the work started on Monday on eight locations inside Tikrit's complex of presidential palaces, where much of the killing is believed to have taken place.

ISIS militants overran Saddam Hussein's hometown last June, capturing around 1,700 soldiers as they were trying to flee Camp Speicher, an air base previously used by U.S. troops on the outskirt of Tikrit. The fall of Tikrit was part of the ISIS onslaught that stunned Iraqi security forces and the military, which melted away as the militants advanced and captured key cities and towns in the country's north and west.

Later, the Islamic State group posted graphic images online that appeared to show its gunmen massacring scores of the soldiers after loading the captives onto flatbed trucks and then forcing them to lay face-down in a shallow ditch, their arms tied behind their backs.

Other videos showed masked gunmen bringing the soldiers to a bloodstained concrete river waterfront inside the presidential palaces complex in Tikrit, shooting them in the head and throwing them into the Tigris River.

After weeks of bitter clashes, Iraqi forces and allied Shiite militias, succeeded in retaking Tikrit from the Islamic State. Their victory was helped by U.S.-led coalition airstrikes, which were not initially part of the operation.

Amin told The Associated Press that at least 12 bodies were exhumed on Monday. Lab tests will be carried out to match them with DNA samples that have already been taken from families of around 85 percent of the victims.

Iraqi state TV showed forensic teams digging in an open area, helped by bulldozers as family members stood nearby. The bodies were tagged with yellow tags while weeping soldiers and relatives lit candles and laid flowers alongside the covered remains. One clip showed unearthed skeletal remains still wearing combat boots.

"The work is continuing and we expect to discover more mass graves in different areas," Amin said. "We expect huge number of bodies to be unearthed."

During their blitz last year, the IS extremists also carried out other mass killings in other areas. One of those massacres was outside the country's second-largest city of Mosul where they forced some 600 Shiite inmates captured from Badoosh prison to kneel along the edge of a nearby ravine and shot them with automatic weapons. The prisoners had been serving sentences for a range of crimes, from murder and assault to nonviolent offenses.

And in Anbar province, the militants shot dead dozens of pro-government Sunni tribal fighters in public areas after capturing their towns.

The Islamic State's onslaught plunged Iraq into its worst crisis since the 2011 U.S. troop withdrawal from the country. The militants have 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 IS 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 threat by the IS militants, who still hold the northern Iraqi province of Ninevah and most of the western province of Anbar, in addition to small areas north of Baghdad in their hands, along with a large swath of land in neighboring Syria.