SEOUL and LONDON -- North Korea fired "multiple" ballistic missiles on Monday, according to South Korea's Joint Chiefs of Staff, as U.S.-South Korea war games began nearby.

The missiles, which were "unidentified," were fired from the North's Hwanghae Province at about 1:50 p.m. local time, the South Korean military said. They were aimed inland, toward the West Sea.

The South Korean military "has increased surveillance and maintaining readiness posture in close cooperation with the U.S.," the Joint Chiefs said.

The annual U.S.-South Korea joint exercises, which are known as "Freedom Shield," were scheduled to begin Monday and run through March 21, according to the U.S. Army.

The training alongside South Korean soldiers will include urban combat, field hospital operations, field artillery exercises, air assault training and air defenses, the Army said in a statement on Monday. The U.S. Marine Corps is also expected to take part in a joint assault exercise.

ABC News' Somayeh Malekian contributed to this report.