Two years after Vice President Mike Pence promised help to Iraq's displaced religious minorities, a U.S. official said American aid has yielded mixed results, with many displaced individuals still unable to return to their hometowns.

The United States has given $380 million to rebuild northern Iraqi communities belonging to Christians, Yazidis, and other religious minorities, according to Hallam Ferguson, Senior Deputy Administrator in the Middle East Bureau for the U.S. Agency for International Development. But almost a million people from the Nineveh Plains, the historic homeland of the predominantly Christian Assyrian people, remain displaced in the wake of a genocide perpetrated by ISIS.

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"The objective of the United States government is for people to return there and in that way, reverse the effects of genocide by ISIS," Ferguson told members of the U.S. Commission on International Religious Freedom at a hearing Thursday. Despite the hundreds of millions of dollars devoted to northern Iraq, Ferguson said, there has been "only modest success in our efforts."

The main problem is a lack of security. Mostly Shia Muslim militia units, known as Popular Mobilization Forces, which emerged to fight ISIS, "continue to operate with impunity" in parts of the Nineveh Plains, said Ferguson.

These militias are formally part of the Iraqi government, but in practice, tend to operate independently, and many express loyalty to Iran. The United States has sanctioned Rayan al-Kildani and Waad Qado, who lead two of these militia units and have impeded the return of displaced individuals in the northern Iraqi towns of Batnaya and Bartella, due to human rights violations.

To counter the influence of these militias, some Christian communities have worked toward securing themselves. Reine Hanna, director of the Assyrian Policy Institute, an Assyrian advocacy group, cited as an example the Nineveh Plain Protection Units, a mostly Christian military organization.

"In the NPU, the United States now has a security partner in the defense of religious freedom," Hanna told the commissioners.

Hanna pointed out that the rate of displaced individuals returning is higher in areas controlled by the Nineveh Plain Protection Units than in other towns. In Qaraqosh, for example, about 70% of those displaced have returned. Fewer than 40% of Christians previously living in Bartella have returned there, while fewer than 1% have returned to Batnaya.

The situation also remains challenging for the Yazidis, another religious minority community. The predominantly Yazidi town of Sinjar "remains highly insecure with significant development problems," said Ferguson. Most Yazidis remain in refugee camps, he added.

Pari Ibrahim, executive director of the Free Yezidi Foundation, a Yazidi advocacy group, said Yazidis are seeking educational and economic opportunity, political representation, and justice for the perpetrators of genocide. But she said Yazidis remain skeptical that help is imminent.

"We always say if there is a chance that we're on the agenda, we are probably the last point on the agenda to be discussed," Ibrahim told the commissioners.

She added that no one trusts either the Iraqi or Kurdish governments, and said the Yazidis "are all looking forward to going and resettling elsewhere."

Hanna echoed Ibrahim's comments about religious minorities distrusting the government.

"There's a complete lack of confidence in the authorities at both the federal and regional levels," said Hanna.