North Korea fired a salvo of short-range ballistic missiles early Thursday, Seoul's military said.

That followed a failed attempt by the nuclear-armed North to put a second spy satellite into orbit on Monday.

"These launches continue the DPRK's reckless behavior, which poses a grave threat to the Korean Peninsula, the region, and international peace and security and undermine the global non-proliferation regime," US State Department spokesman Matthew Miller said in a statement.

DPRK is the acronym for North Korea's full, formal name.

"The United States reiterates that its commitments to the defense of the ROK and Japan are ironclad," he added.

Thursday's launch came hours after Pyongyang sent hundreds of trash-filled balloons across the border to punish South Korea.

The UN Security Council was to meet Friday to discuss the attempt to launch a spy satellite, which was condemned by the United States, Japan and South Korea.

"The DPRK's actions will only intensify its isolation as it undermines stability and security on the Korean Peninsula," Miller said.