Iraq's joint operations command said Monday that it had found a mass grave containing 100 bodies, many of them beheaded, in the village of Hamam al-Alil, south of Mosul. (Iraqi Federal Police)

Iraqi military and police forces said Monday that they have uncovered a mass grave near a small town south of the Islamic State-held city of Mosul. Initial reports say 100 bodies were found, many of them decapitated.

Col. Abdel Rahman Khazali, a spokesman for the Iraqi federal police, said the bodies were discovered Monday at an agricultural college outside the town of Hamam al-Alil, which was recaptured by Iraqi forces over the past three days.

Khazali said medical teams were examining the bodies and trying to determine their identities. “The investigation is just beginning,” he said.

The Iraqi Joint Command issued a brief statement condemning the Islamic State. “Gangs of ISIS militants continue to commit crimes against our people,” the military said.

Fighters from the Islamic State, also known as ISIS, have held the city of Mosul and surrounding towns and villages for more than two years. A massive offensive by combined Iraqi forces, aided by U.S. air support, began three weeks ago.

Villagers in the surrounding area have said that Islamic State militants rounded them up at gunpoint as Iraqi forces advanced and made them walk to Hamam al-Alil. Some who later escaped said that former police and army officers were separated from them and summarily executed as the militants grew suspicious that some were collaborating with the security forces.

As the Iraqi federal police were investigating the mass grave south of Mosul, Kurdish peshmerga forces on Monday recaptured the town of Bashiqa to the northeast.

Peshmerga commanders reported heavy fighting and repeated attacks by Islamic State suicide bombers driving toward troops in vehicles loaded with explosives.

Journalists reported seeing U.S. troops in and around Bashiqa supporting the Kurdish offensive.

In Washington, U.S. officials said that 33,000 Iraqi civilians have been displaced so far by the Mosul operation and have fled to camps and communities outside the city. That number is “lower than initially expected,” a senior administration official said. “But it’s important to keep in mind that the Iraqi and Kurdish forces have not yet reached the most populous areas of Mosul city.”

International aid organizations have estimated that up to 1.5 million civilians have been living in Mosul under Islamic State control. The Iraqi government has urged those who can to remain in their homes during the offensive.

The United States and aid groups estimate that at least 700,000 will be displaced, adding to the 3.2 million Iraqis who have already left their homes since the militants began taking over towns and cities in the summer of 2014. Much of that area has now been recaptured, with Mosul remaining the center of Islamic State control.

Critics, including some U.S. lawmakers, have charged that Mosul is likely to become a humanitarian disaster for which the international community is ill-prepared. But a senior official, one of three who briefed reporters Monday on the condition of anonymity insisted on by the administration, said that with “the advantage of forewarning . . . this situation may be one of the best-prepared responses to a humanitarian emergency that is expected that we have seen, certainly in a long time.”

Emergency food relief for about 1.25 million people has been brought to places where displaced people are expected to come, along with water and hygiene and medical supplies, the officials said. About half the displaced are expected to shelter with friends, relatives and other contacts outside established refu­gee camps, but “we think there will be more than enough shelter in place to meet the needs for a million people by mid-November,” an official said.

While the United States is assisting in preparations for governance in Mosul and for vetting the displaced to make sure militants have not slipped out among them, the officials said those tasks will be handled by the Iraqi and regional Kurdish governments.

Vetting has been problematic in earlier offensives to clear towns and cities farther south, with some Iraqi groups imposing sectarian restrictions.

“We are working closely with them to make sure that their screening is appropriate, that it’s done by government officials, that it’s at planned sites, that it’s standardized . . . [and] open to international monitoring,” a U.S. official said.

“We’ve also deployed mobile protection and legal aid teams and are establishing protection service centers, particularly in those areas where we know families are most likely to flee.”

DeYoung reported from Washington. Loveday Morris in Bartella, Iraq, and Aaso Ameen Shwan in Irbil contributed to this report.