U.S. Indo-Pacific Command said the missiles did not pose an immediate threat to U.S. personnel or territory, or to its allies.
South Korea's presidential Blue House said the weapons North Korea launched were assessed as "a new kind of short-range ballistic missiles."
The location of the drills includes the contested Paracel and Spratly island groups, where Vietnam, the Philippines and others claim territory.
A record 225 "ghost boats" washed up on northern Japanese coasts last year, more than twice the number recorded in the previous year.
