Seoul, South Korea: North Korean soldiers boasted on state television they bombarded a front-line South Korean island with artillery last month as immediate retaliation after the South fired first.

The two Koreas have ramped up their rhetoric since the November 23 attack killed four South Koreans on Yeonpyeong Island near their tense western sea border. North Korea has said it fired after South Korean shells landed in its waters, while the South has said its routine firing drill aimed away from the sea frontier and should not have provoked an attack.

South Korea has staged a series of military drills — including one on Yeonpyeong Island — in a show of force against the North. The South's President Lee Myung-bak, during a visit to front-line troops Thursday, said that South Korea must make "unsparing" retaliation if it suffers another surprise attack.

Border islands

South Korea plans new routine naval firing exercises starting Monday but not on Yeonpyeong and other border islands, according to the Defence Ministry.

The state-run Institute for National Security Strategy in Seoul said in a report issued Sunday that North may even directly invade Yeonpyeong and other Yellow Sea border islands next year, Yonhap news agency reported. Yonhap said the assessment linked the North's belligerence to leader Kim's moves to transfer power to his youngest son but the report didn't elaborate.

Calls to the institute were unanswered yesterday.

Defence chiefs of South Korea and China are to meet in Beijing in February to discuss regional security issues.

South Korea, meanwhile, has decided not to resume calling North Korea its "main enemy" in a defence white paper to be issued in coming weeks, a Defence Ministry official said Sunday requesting anonymity citing department rules.