WHILE an appearance of peace was achieved in an historic 25-point pact between the two rival Koreas last week, officials in Seoul are saying the hard part is yet to come.The pact on nonaggression and reconciliation is just a prelude to a peace treaty, which has eluded the divided and heavily armed peninsula for the 38 years since the end of the Korean War. And its implementation could be even more difficult than the 14 months of on-again-off-again talks needed to craft it. Most worrisome of all, says one high-level security official in Seoul, is the possibility that the Communist regime of North Korea may be using the pact and its difficult implementation as a delay tactic to give it time to finish production of a nuclear bomb, perhaps as early as 1993. "If they simply try to gain time, there will be problems," the official says. The long-anticipated pact lacks specific steps or deadlines for achieving the eased travel restrictions or its stated goals in political, military, and economic areas. In 1972, the two sides issued a joint communique calling for peace and reunification that produced little progress. Another worry is that an image of apparent progress in ties between the two Koreas might soften Japan's stance against normalizing ties with the North and lead to Tokyo giving massive compensation for its 1910-45 occupation of Korea, thus boosting an ailing North Korean economy. Japan, while welcoming the pact, has hinged its ties with the regime in Pyongyang on "substantial progress" in North-South relations as well as international inspection of the North's nuclear complexes. The United States, however, after raising alarm this year over a nuclear reprocessing facility allegedly under way in North Korea, stated that the pact would not alter a decision last month to delay the second scheduled withdrawal of US troops in South Korea. The pact, signed Friday, did not directly tackle the issue of the possibility of nuclear proliferation. Rather, the two parties agreed to meet later this month in the border village of Panmunjom to discuss the matter. Another meeting is set for Feb. 18 to exchange final documents on the pact if it is approved by the two countries' legislative bodies. South Korean news agency Yonhap reported Saturday that the South Korean legislature would not approve the pact until the nuclear issue is resolved. The North has refused to allow international inspection of its nuclear plants, despite signing the 1985 Nuclear Non-Proliferation Treaty. But on Friday, after signing the pact, the nations' two prime ministers issued a statement promising joint efforts for nuclear nonproliferation and a ban on weapons of "mass destruction." North Korean Prime Minister Yon Hyong Muk added: "We now must make another achievement by signing a separate nuclear agreement. We must make the Korean peninsula free of nuclear weapons." But South Korean officials say the North's aim appears to be to drag out the issue as long as possible. In the meantime, however, the pact's most immediate benefit might be an increase in trade between the two nations. The first direct barter trade was allowed by South Korea last April, while trade that went through third countries skyrocketed to $176 million for the first 11 months of this year, up from 19.29 million a year earlier, according to South Korean government estimates. Many South Korean businesses have probed the possibility of trade, even though the North insists that no imported products carry labels indicating its place of origin. The pact also commits each side in writing to not interfering in each other's internal affairs and to respect their different political systems. It also renounces the use of force and calls for a direct phone lines between military authorities. The two sides agreed to set up a liaison office in Panmunjom, a letdown for the South which wanted the offices in each capital. The most emotional aspect of the pact for many South Koreans was its call for personal and press exchanges. Up to now, the Stalinist regime in the North has allowed only limited exchanges. Seoul officials say this is perhaps to forestall unrest: People in the North remain largely ignorant of the outside world. The accord also sets up committees to work on the political, military, and exchange-cooperation sections within one month after the accord is approved.