The Russian military activity, which has increased in frequency and aggression since March, stems from growing cooperation and coordination between Moscow, Tehran and the Syrian government to try to pressure the U.S. to leave Syria, the official said. The official said Russia is beholden to Iran for its support in the war in Ukraine, and Tehran wants the U.S. out of Syria so it can more easily move lethal aid to Lebanese Hezbollah and threaten Israel. The U.S. has seen more cooperation, collaboration, planning and intelligence sharing, largely between mid-level Russian and Iranian Quds force leaders in Syria, to pressure the U.S. to remove troops from Syria, the official added.