SEOUL—North Korea fired artillery at a South Korean island near a disputed western maritime border Tuesday, killing two South Korean marines in a surprise barrage that kindled global worries about worsening relations between the countries.

A South Korean military unit on the island returned fire, while South Korean military officials scrambled fighter jets. At least 16 marines were injured, military officials said. Three civilians were injured, and the island's 1,200 residents were sent scrambling for bomb shelters.

"The...