North Korea fires 100 artillery shells into sea
The drill began shortly before midday (0300 GMT) using land artillery units based at the eastern tip of the Demilitarised Zone that bisects the Korean peninsula, the South's Joint Chiefs of Staff (JCS) said.
It lasted for 30 minutes and about 100 shells, some with a range of around 50 kilometres (30 miles), fell into waters north of the eastern sea boundary, a JCS spokesman said.
None of the shells crossed into South Korean waters.
South Korean border troops were already on heightened alert after a series of short-range ballistic missile tests by the North in recent weeks, including the firing of two Scud missiles into the Sea of Japan (East Sea) on Sunday.
UN resolutions bar North Korea from conducting any launches using ballistic missile technology.
"Today's exercise was seen as a show of force towards our side," a South Korean defence ministry spokesman told AFP.
North Korea often conducts tests and drills as a show of displeasure, and Sunday's missiles were fired after it denounced an upcoming South Korean-US naval exercise.
The annual drill, from July 16-21, involves the US aircraft carrier George Washington, which arrived in the southern port of Busan on Friday.
Previous tests had preceded Chinese President Xi Jinping's state visit to Seoul. They were seen by some analysts as a show of pique at his decision to visit the South rather than the North.
In between the recent launches, Pyongyang has also made several peace overtures to Seoul, including a proposal for both sides to halt all provocative military activity.
The South dismissed the offer as "nonsensical" in the light of the North's nuclear weapons programme and reiterated that the annual joint military drills with the US are non-negotiable.
"The North is showing a two-faced attitude," South Korean President Park Geun-Hye said in a meeting with her advisers on Monday.
Park noted that Pyongyang had kept up the missile tests even while setting up talks with the South on sending athletes to the upcoming Asian Games in the South Korean port city of Incheon.
The talks will be held Thursday at the border truce village of Panmunjom.
"We have to establish a solid defence posture that can resolutely respond to any provocations by the North," Park's office quoted her as saying.
There is no dispute over the eastern maritime boundary, unlike its western counterpart in the Yellow Sea, which Pyongyang refuses to recognise because it was unilaterally drawn by US-led United Nations forces after the 1950-53 Korean War.
Each side complains of frequent incursions by the other across the western border and there were naval clashes in 1999, 2002 and 2009.
In November 2010 North Korea shelled Yeonpyeong island, killing four South Koreans and briefly triggering concerns of a full-scale conflict.
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