North Korea needs to show sincerity to the international community, South Korea's president said Monday, and should "change its mindset" towards its neighbour.

Lee Myung-Bak also urged Pyongyang to open talks with Seoul, as diplomatic efforts to bring the North back to six-nation nuclear disarmament talks intensify.

"Now North Korea must take action and show sincerity to the international community," Lee said in an address marking the anniversary of a popular uprising in 1919 for Korea's independence from Japan's colonial rule.

"For the bright future of South and North Korea, North Korea should change its mindset to regard South Korea only as a partner for economic cooperation."

The frozen dialogue groups the two Koreas, Japan, China, Russia and the United States.

The North has set two conditions for returning to the long-running talks it abandoned last April, a month before staging a second nuclear test.

It calls for UN sanctions to be lifted and wants a US commitment to discuss a formal peace pact, replacing the armistice which ended the 1950-1953 war on the Korean peninsula.

North Korea has in recent months been making peace overtures to the South.

But cross-border military tensions have run high since a Yellow Sea firefight last November left a North Korean patrol boat in flames.

The North's military last week accused South Korean and US troops of planning a surprise attack under the pretext of a joint military exercise. It warned it could respond with atomic weapons.

Pyongyang also announced the detention of four South Koreans for illegally entering the country. Seoul could not confirm the case.