China’s military is increasing harassment of Navy survey ships, activities viewed by U.S. intelligence as the most aggressive since 2001, when a Chinese jet flew into a U.S. EP-3 surveillance plane and set off an international crisis. Chinese diplomats have work to do to keep a confrontation between American and Chinese naval vessels from damaging a relationship that President Barack Obama deems crucial to confronting the world’s toughest crises.

China says a U.S. Navy mapping ship confronted by Chinese vessels Sunday was operating illegally in China’s exclusive economic zone. The United States says Chinese ships surrounded and harassed the Navy vessel in international waters in the South China Sea.