The United States has halted a secretive military intelligence cooperation program with Turkey that for years helped Ankara target Kurdish PKK militants, four U.S. officials told Reuters.

The U.S. decision to indefinitely suspend the program, which has not been previously reported, was made in response to Turkey’s cross-border military incursion into Syria in October, the U.S. officials said, revealing the extent of the damage to ties between the NATO allies from the incident.

The U.S. officials, who requested anonymity due to the sensitivity of the matter, said the United States late last year stopped flying the intelligence collection missions that targeted the Kurdistan Workers Party (PKK), which both the United States and Turkey classify as terrorists.

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The U.S. military had carried out the missions using unarmed drone aircraft, which one official said were flown out of Turkey’s Incirlik air base, where the U.S. military has a significant presence. The base is also a key hub for U.S. spy agencies operating in the region.

Read more: Reuters