Tensions between North and South Korea are again on the rise - with a shooting incident at their heavily fortified border and the cancellation of a joint liberation day celebration.

South Korean military police officer stands guard in front of signboard in demilitarized zone, Tuesday

South Korean military spokesman Ha Doo-chul says North and South Korean soldiers exchanged gunfire across their heavily defended border Monday night - but without injuries.

He says North Korean soldiers fired two or three shots at a South Korean border guard post in the Demilitarized Zone. South Korean soldiers responded immediately, Ha says, firing back six rounds.

This is the first clash this year at the border - where some one million troops face each other and sporadically exchange gunfire.

North and South Korea have remained technically at war since 1953 when they signed a truce in the Cold War conflict but never concluded a peace treaty.

Relations between the two Koreas encountered yet another setback Tuesday, when the communist North canceled joint Liberations Day celebrations scheduled in Pyongyang on August 15. The day marks the Korean Peninsula's freedom from Japanese rule in 1945 at the end of World War II.

North and South Korea have celebrated it together every year since 2000 when Korean leaders held their first and only summit and pledged to work toward reunification.

Pyongyang says it is forced to cancel the ceremony because of massive damage from July's floods. The North also canceled its annual Arirang propaganda festival - which runs from August until October.

Inter-Korean relations have taken on added strain after Pyongyang test fired a series of ballistic missiles July 5 - despite international pressure not to.

South Korea responded by suspending food aid to the impoverished North - which promised to make Seoul "pay" for the decision.

The North has since canceled scheduled reunions of separated Korean families, expelled South Korean workers from a joint resort project, and withdrawn its staff from a Korean liaison office in the city of Kaesong.

Some analysts speculate the flooding may open an opportunity for Seoul to restart food aid - purely on humanitarian grounds. South Korea - which prefers trying to influence the North through engagement - is reluctant to use sanctions as a policy tool.

But Nam Sung-wook, from Korea University and advisor to the South Korean government, says it is much too soon after the missile launches to resume aid.

"From the opinion of international society, like Japan and the U.S.A., the South Korean government cannot change the (aid suspension) policy in just one month," Nam said.

The South Korean government says aid will only be resumed if North Korea returns to six-nation nuclear disarmament talks. North Korea has refused to go back to negotiations since September 2005 when it promised in principle to end its nuclear weapons programs.