Footage captured by U.S. surveillance aircraft over northern Syria has documented several incidents that military officials say may constitute war crimes on the part of Turkish-backed forces there, a U.S. official said.

If verified, the imagery could provide credibility to allegations that the offensive Turkey launched over Western objections last month has resulted in repeated abuses against Syrian Kurds who have been an important U.S. partner against the Islamic State.

A U.S. military official, who spoke on the condition of anonymity to discuss an internal assessment, cautioned that it remains difficult based on the aerial footage alone to definitively determine what occurred in two similar incidents along a major Syrian highway in mid-October.

The imagery suggests that fighters from a group armed and funded by Turkey, which the Pentagon identifies as the Turkish Supported Opposition (TSO), may have apprehended two Syrian Kurds and shot them, the official said, but added that it is not seen as conclusive proof.

The existence of the videos, each titled “Alleged TSO Civ Cas Shooting,” was first reported by the Wall Street Journal.

Recordings of the incidents, which were first viewed in real time, were referred to senior levels of the Pentagon for broader discussions within the administration.

The U.S. military also is tracking other possible abuses by the Turkish-backed forces that have been reported by activists or leaders from the Syrian Democratic Forces (SDF), a Kurdish-led group that U.S. Special Operations troops have worked closely with in Syria for four years.

The Turkish-backed forces are seeking to push the Kurdish group, which Ankara sees as a terrorist threat, back from the area near Syria’s border with Turkey.

News of the footage comes to light on the eve of Turkish President Recep Tayyip Erdogan’s visit to Washington, an event that has intensified criticism of President Trump’s Syria policy. Many Democrats and some Republicans say Trump erred in making an abrupt decision in October to pull U.S. forces back from positions in northern Syria, opening the way for Erdogan to launch a long-promised offensive against Syrian Kurds there.

The Pentagon has been scrambling to adjust to Trump’s decision. While the administration has backed away from an immediate full withdrawal from Syria, it is repositioning forces there and has scaled back its mission against the Islamic State. The militant group no longer holds territory but is believed to retain an important insurgent capability.

The American repositioning has allowed Russian and Syrian government forces to enter areas that they had been shut out of for years, strengthening the position of Syrian President Bashar al-Assad in his quest to reclaim all Syrian territory.

The Turkish offensive, officially named “Operation Peace Spring,” has further disrupted U.S. relations with Turkey, a NATO ally, at a time when Ankara’s decision to acquire air defense systems from Russia already had created significant strain.

Despite those objections, Trump approved Erdogan’s visit after the Turkish government agreed to a U.S.-brokered cease-fire deal that cemented a Kurdish withdrawal from certain areas of northern Syria.