SEOUL (AFP) - North Korea on Saturday launched three short-range missiles into the Sea of Japan, apparently as part of a military drill, South Korea's defence ministry said.

"North Korea launched two guided missiles in the morning and another one in the afternoon," a defence ministry spokesman told AFP.

"The missiles landed in the East Sea (Sea of Japan)," he said.

South Korea was maintaining intense surveillance of the North and keeping a high-level of readiness to deal with any risky developments, he added.

North Korea sometimes launches short-range missiles for tests or as part of military drills.

The launches followed a joint South Korea-US naval exercise this week, slammed by North Korea as a "wanton" provocation and rehearsal for war.

A US strike force led by the nuclear-powered aircraft carrier USS Nimitz participated in the two-day drill that ended on Tuesday.

A US defence official said last week that two North Korean missiles that had been primed for firing as tensions on the peninsula flared in the run-up to the North's national celebrations on April 15 had been moved from their launch site.

US and South Korean officials had been worried that any test of the Musudan medium-range missiles would trigger a fresh surge in tensions, which escalated after the North carried out a nuclear test in February.