Japan and South Korea signed an agreement Wednesday to share intelligence on North Korea's military activities, including its nuclear program.
South Korea's defense ministry said the pact will help "restrain" North Korea's nuclear and missile activity.
"Since we can now utilize Japan's intelligence capability to effectively deal with North Korea's escalating nuclear and missile threats, it will enhance our security interests," the ministry said in a statement.
The agreement allows South Korea and Japan to swap information directly, instead of going through the United States as they had been under a deal signed in 2014.
The new arrangement drew opposition from activists and political parties in South Korea, including those who are demanding the resignation of President Park Geun-Hye.
China, a key ally of North Korea, also objected to the arrangement, saying it will bring an unsafe and unstable element to the region.
"The relevant countries have maintained a Cold War mentality and increased military intelligence cooperation, which will increase opposition and confrontation on the Korean peninsula," Chinese foreign ministry spokesman Geng Shuang said.