Seoul, Aug 17 (DPA) North Korea Monday put its troops on alert as an annual joint South Korean-US war game commenced and accused its neighbour of planning an attack.

North Korea would react to any provocation with a 'merciless and prompt annihilating strike at the aggressors with all offensive and defensive means, including nuclear deterrent', state media quoted the high command of the Korean People's Army as saying.

South Korean's defence ministry played down the statement, saying it was part of Pyongyang's usual posturing in reaction to the annual war games. A ministry spokesman said South Korea could not detect any unusual army movements in the North.

The military exercises, which were scheduled to end Aug 27 involves 10,000 US troops and 56,000 South Korean soldiers.

Earlier Monday, North Korea said it was to reopen its borders for South Korean tourists and family reunions for relatives divided by the inter-Korean border as well as ease access to a jointly operated industrial park in a North Korean border town.