SEOUL — In a significant step toward overcoming lingering historical animosities with its former colonial master, the South Korean government has unexpectedly announced that it will sign a treaty with Japan on Friday to increase the sharing of sensitive military data on two major common concerns: North Korea’s nuclear and missile threats and China’s growing military might.

The announcement triggered a political firestorm in South Korea, where resentment of Japan’s early 20th-century colonization remains entrenched and any sign of Japan’s growing military role around Korea is met with deep suspicion.

The opposition accused President Lee Myung-bak of ignoring popular anti-Japanese sentiments in pressing ahead with the treaty, the first military pact between the two nations since the end of colonization in 1945.

North Korea accused Lee’s government of ‘‘selling the nation out.’’

The accord, the General Security of Military Information Agreement, provides a legal framework for the two nations to share and protect classified and other sensitive data.

It was announced by Cho Byung-jae, the spokesman of the South Korean Foreign Ministry, who said the South Korean ambassador to Tokyo, Shin Kak-soo, and Japan’s foreign minister, Koichiro Gemba, plan to sign the treaty Friday afternoon, immediately after the Japanese Cabinet’s expected approval.

The United States has been urging the two to put their historical hostilities behind them and strengthen military ties, so the trilateral alliance of the nations can deal more efficiently with the threats from North ­Korea. Until now, the governments in Seoul and Tokyo often shared each other’s information on North Korea indirectly, through Washington.