BEIJING (Reuters) - China's Defence Ministry on Wednesday expressed serious concern about South Korea and Japan signing a military intelligence pact to share sensitive information on the threat posed by North Korea's missile and nuclear activities.

The signing of the General Security of Military Information Agreement had originally been expected in 2012, but South Korea postponed it due to domestic opposition.

The case for the neighbors to pool intelligence has increased, however, as North Korea has been testing different types of missiles at a faster rate, and claims it has the capability to mount a nuclear warhead on a missile.

Chinese Defence Ministry spokesman Yang Yujun said the move would add a new unsafe and unstable element to northeast Asia and smacked of a Cold War mentality.

"China's military expresses serious concern about this," Yang told a monthly news briefing, adding to previous opposition to the deal from the Chinese Foreign Ministry.

"We will make all necessary preparations, earnestly perform our duties and fulfill our mission, resolutely protect the country's security interests and resolutely protect regional peace and stability," he added, without elaborating.

Beijing is North Korea's most important supporter despite Chinese anger at its missile and nuclear tests in defiance of UN Security Council resolutions. Earlier this month, the Chinese Foreign Ministry said the agreement would add to tension on the Korean peninsula.

Reclusive North Korea and the rich, democratic South are technically still at war because their 1950-53 conflict ended in a truce, not a peace treaty. The North regularly threatens to destroy the South and its main ally, the United States.

China has also been upset with South Korea for agreeing to host an advanced U.S. anti-missile system, saying it threatens China's strategic security.

South Korea went ahead with the deal despite opposition from some political parties and a large section of the public, who remain bitter over Japan's actions during its colonial rule of Korea from 1910 until the end of World War Two.

(Reporting by Ben Blanchard; Editing by Nick Macfie)