Around 44,000 Japanese and US troops began week-long joint military drills around Japan on Friday, 10 days after North Korea attacked a South Korean island, dpa reported.
About 60 vessels and 400 planes from the two countries' armed forces were taking part in the exercises in the Sea of Japan and near Okinawa island, which come two days after a joint drill between the United States and South Korea ended.
The South Korean Defense Ministry has dispatched observers to the drills for the first time. Japan sent observers to US-South Korean exercises in July.
North Korea on November 23 shelled Yeonpyeong island, near the disputed border between the Koreas in the Yellow Sea, killing two marines and two civilians.