The United States is investigating what it considers credible reports that weapons it sold Turkey have been misused in the country’s military operation in northern Syria, Pentagon officials told CNN.

The reports say Turkey has breached its agreements for the end-use of the weapons and may have transferred them to Syrian rebel allies suspected of committing war crimes during the operation.

Ankara launched Operation Peace Spring against Kurdish-led forces in northern Syria that it views as terrorists due to their links to insurgents in Turkey. The offensive was launched on Oct. 9 after U.S. President Donald Trump withdrew troops that had been deployed alongside the Syrian Kurds, who had fought the Islamic State with U.S. backing for years.

Turkish forces advanced quickly into northern Syria with the aid of Syrian rebel groups it has backed throughout the eight-year conflict. Video footage believed to have been shot by the rebel groups is viewed by many as evidence that they have committed war crimes during the operation, including the ambush and murder of Hevrin Khalaf, a Syrian Kurdish politician.

If Turkey transferred military equipment and weaponry bought from the United States to those groups, this would breach standard user agreements on the use of that equipment, CNN said.

"Consistent with our end-use monitoring agreements, the United States always investigates credible allegations," Pentagon spokesperson Lt. Col. Carla Gleason told CNN on Wednesday.

As well as being prohibited from transferring weaponry to third parties by the agreements, Turkey could be held responsible if its Syrian rebel proxies, rebranded as Syrian National Army (SNA), are found to have committed war crimes.

"By all of the rules of -- as we understand international affairs, if you have a force that you support, supply, provide air support for and, essentially, effectively command and control, and that is the situation of the government of Turkey over these Turkish-supported organisations, then you bear at least part of the responsibility,” CNN quoted a senior State Department official as saying.