HONG KONG/MANILA, April 15 (Reuters) - China is employing ships and a barrier to tighten control of the entrance to the Scarborough Shoal in the South China Sea amid roiling ​tension with the Philippines over the disputed feature, satellite imagery obtained by Reuters shows. Scarborough is one of Asia's most hotly disputed maritime sites, where ‌some diplomats and analysts fear long-running frictions and confrontations could degenerate into armed conflict.