SEOUL, SEPT. 18 -- The South Korean government and ruling party are making new diplomatic overtures to North Korea in a bid to ease tensions and rebuff opposition challenges to the government's hard-line stance toward Pyongyang, according to western diplomats and political analysts.

Reversing a longstanding policy, Foreign Minister Choi Kwang Soo said yesterday that South Korean diplomats would now be encouraged to have informal contacts with their counterparts from the North. In a separate move, Roh Tae Woo, the ruling Democratic Justice Party's presidential candidate, called a week ago for a "fresh and bold" diplomatic initiative to unify the Korean Peninsula, divided roughly along the 38th Parallel since 1945.

By offering to increase official contacts, the South Korean government is extending a somewhat limited olive branch to the government of Kim Il Sung, the diplomats said.

The diplomats added that Seoul wants to avoid a potentially explosive situation in which Pyongyang boycotts the 1988 Seoul Olympics and, in the worst-case scenario, tries to disrupt the games through terrorism.

South Korea, which is spending $3.1 billion on the Olympics, has staked its international prestige on holding trouble-free Games. North Korea, increasingly isolated in its bid to cohost the games, recently rejected an offer from the International Olympic Committee to host five events. Although tripartite talks between the committee, Seoul and Pyongyang are expected to continue, officials here expressed pessimism about the chances of reaching an accord.

Political analysts and diplomats here warned that South Korean initiatives to the North usually contain a fair dose of propaganda. According to a western diplomat, "You can characterize {Choi's announcement} as propaganda, because it has that function too, but there is some substance." A West European diplomat, noting that Choi's statement came exactly one year before the Olympics open, described it as "a limited gesture of conciliation."

Last year North Korea broke off most official contacts with the South, including Red Cross, economic and parliamentary talks, after Seoul held its annual joint military exercises with U.S. troops. However, Pyongyang opted to continue the Olympic negotiations.

The Reagan administration, in a relaxation of its tough stance toward Pyongyang, announced earlier this year that U.S. diplomats would be allowed to have informal contacts with North Korean diplomats. A U.S. official here said South Korea's decision to adopt a similar policy was a "positive step."

Meanwhile, Roh, who returns tomorrow from a weeklong visit to Washington and Tokyo, appears intent on distancing himself from the relentlessly hard-line policy pursued by President Chun Doo Hwan and previous regimes. One of the central planks of the opposition Reunification Democratic Party is to warm up relations between the two Koreas, which remain arch-enemies 35 years after the Korean War.

"I feel a sense of mission in connection with the early realization of the reunification of the fatherland," Roh said last week. "A fresh and bold Nordpolitik and unification policy should be adopted for that purpose."

While visiting Washington, Roh offered to allow North Korea's Kim to visit Seoul and speak on national television if Roh could visit Pyongyang and make a broadcast on the North Korean airwaves. The proposal recalled Chun's offer of reciprocal summits a few years ago.