North Korea has test fired seven short-range missiles into the sea, South Korea said Friday, in its latest launch of missiles during joint military drills between South Korea and the United States.

A South Korean defense ministry official, who could not be identified because of official policy, said the surface-to-air missiles launched from North Korean western coast late Thursday flew into waters off the eastern coast.

North Korea routinely test launches missiles, rockets and artillery but the latest launch came as South Korea and the U.S. conduct the annual drills that Pyongyang calls an invasion rehearsal. Seoul and Washington say the drills are for defense.

North Korea fired two short-range missiles on the first day of the drills earlier this month and warned of "merciless" strikes against South Korea and the U.S. The military exercises will continue until late April.