A Syrian military helicopter was shot down over the last major rebel bastion in northwest Syria on Friday, the second such incident in a week of high tensions with Turkey. The attack in a region where Turkish troops and Russian-backed government forces have engaged in multiple clashes came as Washington urged Ankara to look to its Western allies in light of Moscow’s actions. Syrian and Russian forces have pressed a deadly offensive against the shrinking pocket in the country’s northwest since December, claiming the lives of nine civilians on Friday.