SEOUL, DEC. 3 -- The disappearance Sunday of a Korean Air jetliner with 115 passengers aboard put North Korea in the center of South Korea's presidential campaign today, although the cause of the apparent crash remained a mystery.

Until now, the question of the opposition's anticommunist credentials on this divided peninsula has not been, at least openly, a major issue in the Dec. 16 election. But the disappearance of KAL 858, which investigators believe may be the work of North Korean agents, seemed to be changing that.

Ruling party candidate Roh Tae Woo, citing the plane incident, continued his move away from the unexpectedly flexible policy toward North Korea he had adopted early in the campaign. Roh said the incident demonstrates the North Korean threat and the danger of candidates who do not recognize that threat.

Opposition politicians accused the ruling party of trying to exploit the tragedy for political gain.

The jet disappeared over Southeast Asia while flying from the Middle East to Seoul. South Korean airline and government officials believe the jet was destroyed by a time bomb.

{South Korea's Yonhap News Agency and three Seoul newspapers reported that a Burmese fishing boat spotted what appeared to be the wreckage of an airplane in the Andaman Sea near an island about 25 miles southwest of Tavoy in southwestern Burma, according to United Press International. But a South Korean Foreign Ministry official said he could not confirm the report.}

The airplane incident appeared to accelerate Roh's move toward the right in the campaign. Two months ago, he was promising to open the way to dialogue between the bitterly divided Koreas.

Hard pressed by opposition candidates Kim Young Sam and Kim Dae Jung, however, Roh has increasingly stressed the point that only he can stand up to the communist north. "The opposition parties include militant radicals who do not hesitate in their private gatherings to advocate a violent revolution," Roh said yesterday.

Kim Dae Jung, the strongest advocate of improved dialogue with the north, could lose votes if North Korea is shown to have played a role in the crash. Kim said today voters are mature enough not to allow the incident to affect their vote.

Kim Young Sam's Reunification Democratic Party also accused Roh's campaign of "scheming to use the tragic Korean Air lines incident."