BEIRUT -- As the U.S.-led coalition ratchets up military operations in Syria ahead of a long awaited assault on the Islamic State of Iraq and Syria (ISIS) group's de facto capital Raqqa, the legacy of an Iraqi train-and-equip program, though it has had some success, is also marked by allegations of abuse and $1 billion in unaccounted for weapons -- highlighting the perils of empowering local forces in the fight against ISIS.

Weapons, training and airstrikes provided by the U.S.-led coalition have propelled ground forces in both Iraq and Syria, allowing Iraq's military, Iraqi Kurdish fighters and Syrian Kurdish fighters to retake some 21,235 square miles of territory from the extremists in the nearly three-year long fight.

However, many in both Iraq and Syria are concerned about how the forces made powerful by the coalition will leverage their influence and arms once ISIS has been vanquished.

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The Trump administration's decision earlier this month to provide Syria's Kurds with more advanced weapons has sparked concerns among the various players in Syria's complicated battlefield. U.S. officials have said new weapons to be supplied would include heavy machine guns, ammunition, mortars and possibly TOW anti-tank missiles.

U.S. officials acknowledged the move would unsettle NATO ally Turkey, which views some Kurdish groups as terrorists.

Coalition spokesman Col. John Dorrian said the weapons will not be reclaimed after the specific missions are completed but the U.S. will "carefully monitor" where and how they are used. "Every single one" of the weapons will be accounted for and the U.S. will "assure they are pointed at" ISIS, he said.

But opposition fighters battling Syrian forces amid the country's six-year civil war -- some of them backed by Turkey -- say there is simply no guarantee the weapons will not be directed against them or others. U.S.-backed Kurdish groups have often clashed with Turkey-backed groups in northern Syria, where a multitude of factions are jostling to maintain various zones of influence.

The coalition has already demonstrated an inability to track weapons in Iraq, a much less complex and unstable battlefield than in Syria. Amnesty International released a report this month detailing a 2016 Department of Defense audit stating that $1 billion in weapons provided to Iraqi forces for use in the ISIS fight are now unaccounted for.

The coalition could have worked closer with the Iraqi government to ensure the weapons were accounted for, said Patrick Wilcken, a researcher with Amnesty and an author of the report. But in Syria, he explained, it will be "almost impossible to avoid leakage and diversion of arms" provided by the coalition to fighters there.

"The Coalition takes all reasonable efforts to maintain accountability of equipment divested to the government of Iraq to fight" ISIS, coalition spokesman Col. Ryan Dillon told The Associated Press. He added that since the 2016 audit referenced in the Amnesty report, "all deficiencies identified in that report have been corrected." Iraqi commanders must sign for all equipment they receive and the coalition then continues to monitor them "for future vetting purposes" and on the battlefield.

This month an explosive report from Germany's Der Spiegel magazine detailed allegations of torture, rape and killings of ISIS suspects at the hands of Iraq's Emergency Response Division, an Interior Ministry unit that has played a lead role in the operation to retake Mosul with close coalition backing.

Iraqi soldiers, Kurdish forces and local policemen have all been accused of carrying out mass extrajudicial detentions of men and boys fleeing military operations against ISIS, according to reports by Human Rights Watch and the Associated Press. Syrian Kurdish forces backed by the coalition have also been accused of abuses against Sunni Arabs, according to human rights organizations and Syrian opposition activists.

Other armed groups -- notably Iraq's mostly Shiite paramilitary forces who do not receive direct U.S. assistance of any kind -- have been accused of much more widespread human rights abuses than the forces backed by the U.S.-led coalition.

In Iraq, the numerous abuse allegations are in part due to the fact the government has yet to hold a single person accountable for major rights violations in the fight against ISIS, said Human Rights Watch Iraq researcher Belkis Wille. "That is why you will continue to see (human rights) failing in the Iraqi security forces," she said.

In Syria, it is unclear how allegations of abuse will be probed, as the coalition is not working with a partner government there.

The U.S. human rights law known as the Leahy amendment prohibits the Department of Defense from providing military assistance to foreign military units that violate human rights, and in March 2015, the Iraqi Emergency Response Division (ERD) was disqualified from receiving U.S. equipment and training, coalition spokesman Dillon told the AP.

But he said the law does not prevent the U.S. from working with the ERD to help ensure a coordinated effort among different elements of the Iraqi security forces. The U.S.-led coalition has shared intelligence with the unit and conducted airstrikes to facilitate their military operations.

"Any violation of the law of armed conflict would be unacceptable and should be investigated in a transparent manner and those deemed responsible held accountable," he said.

Iraq's Kurdish forces known as the peshmerga -- who have received some of the most extensive support from the coalition, including training, arms and air support -- have been accused of destroying Arab property and forcing Arab residents out of dozens of villages retaken from ISIS.

The Associated Press visited one such village outside Kirkuk where Arab residents said Kurdish forces labeled their homes as "confiscated," seized identification documents and reduced buildings to rubble. Iraq's Kurdistan regional government denies the claims, saying ISIS fighters destroyed the houses as they retreated.

In northern Syria, Sunni rebels are concerned that Syrian Kurdish forces will mirror the actions of Iraq's peshmerga and use the fight against ISIS to expand the land under their control, ultimately creating a separate state by pushing out ethnic Arabs after the ISIS fight. Amid the chaos of the Syrian civil war, the Kurds have already created an autonomous Kurdish zone in northern Syria.

An Amnesty International fact-finding mission to northern Syria in 2015 uncovered forced displacement of Arab residents carried out by Kurdish forces that the group said amounted to war crimes. The report detailed the deliberate demolishing of civilian homes as well as razing and burning entire villages previously captured by ISIS. The Kurds have rejected the claims.

Col. Abdul-Razzak Ahmad Freiji, a Syrian army defector who is now with Turkey-backed rebels in the country's north, said news of U.S. arms to Syrian Kurdish fighters exacerbates his concerns.

After the fight with the Islamic State group is over, Freiji said, "these weapons (will be directed) against us."