North Korea has fired "several" banned ballistic missiles that flew about 1000km into waters off its east coast, South Korea's Military says, in an apparent reaction to huge Military Drills by Washington and Seoul that Pyongyang insists are an invasion rehearsal.

Japanese Prime Minister Shinzo Abe said the reclusive country fired four ballistic missiles, three of which fell into Japan's exclusive economic zone.

Tokyo had "lodged a stern protest with North Korea," Abe told reporters.

The launch was "a grave threat to our country's national security," the government's spokesman Yoshihide Suga told a news conference.

It was not immediately clear what type of missile was fired or the exact number.

The South's Joint Chief of Staff said in a statement that Monday's launches were made from the Tongchang-ri area in North Pyongan province.

Pyongyang has staged a series of missile test-launches of various ranges in recent months. The ramped-up tests come as leader Kim Jong Un pushes for a nuclear and missile programme that can deter what he calls US and South Korean hostility toward the North.The area is the home of the North's Seohae Satellite Station where it has conducted prohibited long- range rocket launches in recent years.

Seoul and Washington call their military drills on the Korean Peninsula, which remains in a technical state of war because the 1950-53 Korean War ended with an armistice and not a peace treaty, defensive and routine.

The North hates the military drills, which run until late April and which analysts say force its impoverished military to respond with expensive deployments and drills of their own.

An unidentified spokesman for the North's General Staff of the Korean People's Army said last week that Pyongyang's reaction to the southern drills would be the toughest ever but didn't elaborate.