More than 14,000 US personnel are participating in 10 days of drills, which will include the aircraft carrier USS Ronald Reagan, the guided-missile destroyers USS Stethem, USS Chafee and USS Mustin, and a maritime patrol and reconnaissance squadron. The annual drill "is created to increase the defensive readiness and interoperability of Japanese and American forces through training in air and sea operations", AFP reported, citing the statement.
North Korea has conducted dozens of missile tests this year and its largest and biggest nuclear test on September 3.
"The exercise follows more than a week of scenario-based training ashore", it said.
North Korea conducted its sixth nuclear test this year and test-fired a series of advanced missiles, including intercontinental ballistic missiles. "But our military is keeping a full readiness posture against the possibility of North Korea's provocations", a South Korea statement said.
The three told Japanese officials they came with 12 other crew members from North Korea to fish and their boat capsized just as they were heading home, Nanjo said.
After a meeting with the Commander of the US Pacific Command Admiral Harry Harris on Thursday, Japanese Prime Minister Shinzo Abe had stressed on the need to reinforce the capacity of deterrence and joint response by Tokyo and Washington owing to a worsening security situation in the region.
Satellite images taken this month of a North Korean naval shipyard indicate Pyongyang is pursuing an "aggressive schedule" to build its first operational ballistic missile submarine, a United States institute reported on Thursday.
The report said that throughout 2017 there had been continued movement of parts and components into and out of two parts yards adjacent to the constructions halls in the center of the shipyard.