SEOUL — Faced with mounting political pressure at home, the South Korean government Friday abruptly postponed the signing of its first military cooperation pact with Japan since World War II.

Now the fate of the agreement has become uncertain as South Korea’s politicians look ahead to a presidential election this year.

The government and opposition parties agreed Friday to convene the National Assembly on Monday, and the Foreign Ministry said the session would give it a chance to explain and seek parliamentary support for the pact.

The opposition and other government critics bitterly accused the government of trying to rush the agreement through without adequate public debate.

The deal had been hailed as a significant step by the two governments. Both countries have been struggling to overcome the lingering historical bitterness between their peoples and cooperate more closely over mutual security concerns 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.

The United States has urged the governments in Tokyo and Seoul to increase cooperation.

But the pact triggered 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 US pressure and called him ‘‘pro-Japanese,’’ the worst accusation a South Korean politician or political party could face, especially in an election year.

Friday’s postponement provided Washington with yet another reminder of how sensitive and unpredictable the relations between its two main Asian allies can be and how difficult it is to persuade them to cooperate more closely within what the United States envisions as a trilateral alliance.

Earlier Friday, despite the outcry, the South Korean Foreign Ministry had issued a news release reconfirming that the deal would be signed later in the day as soon as the Japanese Cabinet approved it. The Cabinet did, and Japan’s foreign minister called the agreement a ‘‘historic event.’’

But Seoul requested a postponement at the last minute.

Only hours before, Lee Hahn Koo — the floor leader of the governing New Frontier Party and close ally of Park Geun Hye, the party’s leading contender for the presidential nomination — called 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.’’