The unspecified projectile was fired into waters off the east coast of the Korean Peninsula, the agency said, quoting the Joint Chiefs of Staff, which it said gave no details.

Washington and Seoul have ramped up defence cooperation in the face of growing military and nuclear threats from the North, which has conducted a series of increasingly provocative banned weapons tests in recent months.

The new show of force came hours before the leaders of South Korea and Japan were to meet in Tokyo, with Pyongyang's missile and nuclear programs among items on their agenda, and as South Korea and the United States proceed with joint military exercises. North Korea says such maneuvers are a dress rehearsal for invasion.

On Tuesday North Korea fired two short range ballistic missiles in its first launch since the maneuvers -- the biggest in five years -- began, according to the South Korean military.

And on Sunday Pyongyang fired two strategic cruise missiles from a submarine in apparent protest over the US-South Korea exercises.

Known as Freedom Shield, the drills started Monday and run for 10 days.

In a rare move, Seoul's military this month revealed the two allies' special forces were staging military exercises dubbed "Teak Knife" -- which involve simulating precision strikes on key facilities in North Korea -- ahead of Freedom Shield.

The Freedom Shield exercises focus on the "changing security environment" due to North Korea's redoubled aggression, the allies have said.