“We have decided to push for the signing after consultations with the National Assembly,” the Foreign Ministry said in a statement. The deal had been hailed as a significant step by the two governments. Both countries have been struggling to overcome the historical bitterness between their peoples and to cooperate more closely on mutual security matters in the region. The pact provides a legal framework for South Korea and Japan to share and protect classified military data so they can deal more effectively with the threats posed by North Korea’s nuclear and missile programs and China’s growing military influence.

Washington has urged the governments in Tokyo and Seoul to increase cooperation.

But the pact set off an uproar in South Korea, where resentment of Japan’s early 20th-century colonization remains raw and the public regards any sign of Japan’s growing military role with deep suspicion. The opposition accused President Lee Myung-bak of succumbing to American pressure and called him “pro-Japanese,” the worst accusation a South Korean politician could face, especially in an election year. Friday’s postponement provided Washington with yet another reminder of how delicate and unpredictable the relations between its two main Asian allies can be and how difficult it is to persuade them to cooperate within what the United States envisions as a trilateral alliance.

Earlier on Friday, despite the outcry, the South Korean Foreign Ministry issued a news release reconfirming that the deal would be signed later in the day, after the Japanese cabinet approved it.

The Japanese cabinet did, and Japan’s foreign minister called the agreement a “historic event.”

But Seoul requested a postponement at the last minute.

Hours before, Lee Hahn-koo — the floor leader of the governing New Frontier Party and an ally of Park Geun-hye, the party’s leading contender for the presidential nomination — called the foreign minister, Kim Sung-hwan, to demand a postponement. Jin Young, the chief policy maker of the party, said the deal “contradicted the people’s sentiments.”