SEOUL—North Korea fired two more short-range missiles into the sea off its eastern coast on Monday, extending its series of launches into a third day in a possible extended test of a new rocket launcher, South Korea's defense ministry said.

North Korea confirmed its weekend launches but said they were part of defensive drills. Analysts noted that short-range missile firings are relatively routine for North Korea and said Seoul's announcements appeared to be aimed at highlighting the threat from the North and bolstering support...