SEOUL—North Korea warned Friday that it would attack South Korea more violently than it did last month if Seoul proceeds with plans to test-fire artillery from the island Pyongyang shelled.

The statement raises the stakes on what was already seen as a risky test of the fortitude of both Koreas to dispute the inter-Korean maritime border. Officials in the U.S., Russia and at the United Nations Friday expressed concern that the situation on the Korean peninsula could escalate out of control.

The...