Japan and South Korea on Wednesday signed an agreement to share defence intelligence about North Korea, despite protests from opposition parties and activists in Seoul and strong criticism from China.

South Korea's defence ministry said the accord was necessary in the face of growing military threats from Pyongyang, which has conducted two nuclear tests and more than 20 missile launches this year.

"It is ready to conduct additional nuclear tests and missile launches at any time," the ministry said in a statement.

"Since we can now utilise Japan's intelligence capability to effectively deal with North Korea's escalating nuclear and missile threats, it will enhance our security interests."

Japan's foreign ministry said in a statement the military agreement would allow the two governments to "share information even more smoothly and swiftly".

But China, already angry at South Korea's planned deployment of a US missile defence system, sharply criticised Seoul and Tokyo for what it termed a "cold war mentality".

The agreement "will aggravate the situation in the Korean peninsula and bring new unsecure and unstable factors to Northeast Asia", said foreign ministry spokesman Geng Shuang at a regular briefing in Beijing.

"While conducting military cooperation, relevant countries should respect the security concerns of regional countries and do more things for peace and development, not the opposite."

China says Seoul's earlier decision to deploy the THAAD missile defence system will increase the risk of military conflict in the region.

Seoul and Tokyo currently use their mutual ally Washington as an intermediary when sharing military intelligence on Pyongyang, under a deal signed in 2014.

The new intelligence-sharing agreement is also controversial in South Korea, where memories of Japan's harsh 1910-45 colonial rule still mar relations with Tokyo.

South Korea and Japan were on the verge of signing an intelligence-sharing deal in June 2012, but Seoul backtracked at the last minute in response to a public outcry.

Noting Tokyo's surveillance assets and geographic location, South Korea's defence ministry said the deal would be a "big help" in better analysing Pyongyang's nuclear and missile programmes and collecting more intelligence about its submarine-launched ballistic missiles.

North Korea has slammed the military pact, labelling it as "a dangerous act" that would further raise already-high tensions on the Korean peninsula and open a door to Japan's "re-invasion".

The contentious issue comes as South Korean President Park Geun-Hye faces growing calls for her resignation over a widening corruption and influence-peddling scandal that has sparked huge street demonstrations.

The deal has been fiercely opposed by South Korean opposition parties and activists, who point to Seoul's failure to seek public support and historical sensitivities.

South Korea's main opposition party has called the deal "unpatriotic and humiliating" and threatened to impeach Defence Minister Han Min-Koo if the agreement was pushed through.