WASHINGTON (AP) " Defense Secretary Jim Mattis is warning against dangerous flights by Russian fighter jets over what is considered a safe zone in Syria, questioning whether recent close calls were mistakes or deliberate.

Mattis told Pentagon reporters Friday that the U.S. and Russia are still using a phone line to coordinate flight movements over crowded Syrian skies as the U.S.-led coalition goes after remaining Islamic State group militants.

Two Russian Su-25 fighters flew through an unofficial line separating Russian and American air forces in eastern Syria on Wednesday coming dangerously close to U.S. aircraft.

U.S. Air Force F-22 fighter jets launched warning flares in response.

Mattis says he doesn't expect perfection in flights but also doesn't want dangerous maneuvers. He says it's not clear if it was deliberate or just sloppy flying.