North Korea has deployed surface-to-air missiles close the border with South Korea, the South Korean state news agency Yonhap reported.

The news comes just as the US and South Korea are starting four days of joint military in the waters off the Korean coast, in an effort to deter North Korea from launching further attacks across its border with the South after several days ago North Korea shelled the South Korean island of Yeonpyeong, killing two marines and two civilians.

According to the Yonhap report, which cites an anonymous senior South Korean military officer, the North Korean forces deployed the surface-to-air missiles on Tuesday after the shelling of Yeonpyeong in order to target South Korean fighter jets flying close to the so called Northern Limit Line, the demarcation border between the two states.

An American nuclear-powered aircraft carrier, the USS George Washington, is leading the naval drills, which have been designed to take place about 100 km south of the Northern Limit Line.

Earlier Saturday, China launched a flurry of diplomatic activity to ease tensions on the Korean peninsula after North Korea's shelling of the island. China has warned it opposes any "unilateral military act" in the area without its permission, referring to the U.S.-South Korean drills, reported the VOA.

The Yeonpyeong incident has been described as the most serious act of tension since the end of the Korean War in 1953. International concerns with respect to North Korea are growing, as what is known in the West as a rogue state has developed nuclear weapons in the past years.

Analysts have linked the increased level of hostility on part of the North Korean regime with the succession process of the transfer of power North Korean dictator Kim Jong-Il to his third son, 27-year-old Kim Jong-Un.