The U.S. has declined to tell the South Korean military if one of its Aegis destroyers is plowing the East Sea with a view to intercepting a long-range ballistic missile North Korea is allegedly planning to launch.

Washington normally gives Seoul due notice when an aircraft carrier or Aegis ship is headed into Korea’s maritime operational zone, but it sometimes keeps quiet about ships it sends into international waters off North Korea. Military insiders say there is a good chance the U.S. already has an Aegis vessel in the East Sea. Meanwhile, the U.S. started the largest military exercise in 10 years. The operation codenamed “Valiant Shield” takes place in waters surrounding Guam, with three aircraft carriers joining drills from Monday until Thursday. A spokesman of the U.S. Forces in the Pacific said the possibility that the missile situation in North Korea could have an effect on the drills cannot be ruled out.