SEOUL—A new nuclear test by North Korea could have a domino effect by providing its neighbors with a pretext to arm themselves with nuclear weapons, South Korean President Park Geun-hye said.

In a bleak assessment of the implications of North Korea following through on a recent threat to stage a fourth nuclear weapons test, Ms. Park told The Wall Street Journal that Seoul continues to keep a door open to Pyongyang but that a new bomb test would have a "huge impact" on the regional security landscape.

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