By Heekyong Yang and Joori Roh
SEOUL (Reuters) -North Korea fired 100 more artillery shells off its west coast on Wednesday, South Korea's military said, just hours after it launched hundreds of shells into the sea off its east and west coasts in what it called a grave warning to South Korea.
North Korea has been carrying out weapons tests at an unprecedented pace this year, firing a short-range ballistic missile and hundreds of artillery rounds near the heavily armed inter-Korean border on Friday.
On Monday, South Korean troops kicked off their annual Hoguk defence drills designed to boost their ability to respond to North Korea's nuclear and missile threats.
Pyongyang has angrily reacted to the South Korean and joint military activities, calling them provocations and threatening countermeasures. Seoul says its exercises are regular and defence-oriented.
North Korea fired the latest shots at around 12:30 p.m. (0330 GMT), South Korea's Joint Chiefs of Staff said in a statement, urging Pyongyang to halt acts threatening peace and security in the region.
In Washington, a spokesperson for the U.S. State Department said, referring to North Korea by the initials of its official name: "We are aware of these reports. We call on the DPRK to cease all provocative and threatening actions."
Earlier, a spokesman for the General Staff of the North's Korean People's Army (KPA) said the latest move was in response to South Korea's firing of over 10 shells of multiple rocket launchers near the frontline between 8:27 a.m. and 9:40 a.m.
"Our Army strongly warns the enemy forces to immediately stop the highly irritating provocative act in the frontline areas," the KPA official said.
Wednesday's exchange of firings comes shortly after the North fired some 100 shells into the sea off its west coast and shot a further 150 rounds off its east coast on Tuesday night.
North Korea said later the shots were designed to send a "serious warning" and "powerful military countermeasure" to South Korea.