South Korea president in surprise visit to island near North Korea The de-facto maritime boundary between the two Koreas - the Northern Limit Line - is not recognised by Pyongyang, which argues it was unilaterally drawn by the US-led United Nations forces after the 1950-53 Korean War. Lee's visit came the same day that South Korea announced it would begin next week an annual, large-scale military exercise aimed at countering threats from North Korea. The week-long Hoguk exercise beginning October 25 will involve 240,000 army, navy, air force and marine corps personnel, along with police officers, a spokesman of the Joint Chiefs of Staff told AFP. About 500 US soldiers will also take part. Some 28,500 US personnel are stationed in the South - a legacy of the Korean War that ended with a ceasefire but not a peace treaty, leaving the two Koreas still technically at war. "The exercise will feature drills against infiltration, regional provocations or an all-out war by North Korea," a defence ministry spokesman told reporters. Tensions on the Korean peninsula have been raised in the past month by a series of maritime border confrontations, defections and a new US-South Korean deal to nearly triple the range of the South's missile systems. There are also widespread concerns in the South that Pyongyang might do something provocative in the run up to South Korea's presidential election in December.