SEOUL, South Korea -- North Korea fired five short-range projectiles into the sea on Monday, Seoul officials said, in a continuation of weapon launches it has carried out in an apparent response to ongoing South Korea-U.S. military drills it sees as a provocation.

The projectiles launched from a site near the northeastern city of Hamhung flew about 200 kilometres before landing in waters off North Korea's east coast, South Korea's Joint Chiefs of Staff said.

The firings came three days after Seoul said North Korea launched its first medium-range missile into the sea since early 2014, ignoring UN resolutions against such tests.

South Korea's Yonhap news agency initially reported that a missile had also been launched on Monday, but later changed that to say projectiles had been fired.

The firings appear to be North Korea's response to annual springtime U.S.-South Korean military exercises that it says are a rehearsal for an invasion. Earlier this month, North Korea fired several short-range missiles and artillery shells into the sea and threatened pre-emptive nuclear strikes against Washington and Seoul.

This year's drills are the largest ever, and come after North Korea conducted a nuclear test and a long-range rocket launch earlier this year, leading the UN Security Council to impose its toughest sanctions on the country in two decades.

North Korean leader Kim Jong Un recently ordered tests of a nuclear warhead and ballistic missiles capable of carrying such warheads. He issued the orders while overseeing what state media called a successful simulated test of a re-entry vehicle aimed at returning a nuclear warhead to the atmosphere from space so it could hit its intended target. The re-entry vehicle is considered one of the last major technologies North Korea must master to develop long-range missiles equipped with nuclear weapons capable of reaching the U.S. mainland.

Analysts in South Korea said the medium-range missile launch last Friday may have been a test of the re-entry technology. North Korean state media have not commented on the reported launch.

South Korean defence officials say North Korea doesn't yet have functioning intercontinental ballistic missiles.