South Korea has begun a series of major live-fire exercises despite warnings from the North against conducting them.
The drills are spread off South Korea's east, west and south coasts; the most contentious area - the disputed western sea border - has been largely excluded.
It comes two weeks after North Korea shelled a Southern island in response to previous military exercises there, killing four South Koreans.
North Korea has warned that it viewed any such activity as provocative.
South Korea's new defence minister, installed after the attack on Yeonpyeong island, has said he would use air strikes against the North if it attacks civilians again.
President Barack Obama has urged China to work with the US to send "a clear message" to North Korea that its provocations against South Korea are "unacceptable".
Mr Obama made the appeal in a telephone call to President Hu Jintao.
The US is to host talks on Monday with Japan and South Korea on ways to deal with the North.
All three nations have so far failed to back China's call for a resumption of stalled six-party talks on ending North Korea's nuclear activities.
China is North Korea's most important ally and has been pressured by the US and others to use its influence to rein in Pyongyang.
Washington, Seoul and Tokyo oppose a return to the negotiations until the North shows it is serious about nuclear disarmament. BBC News
