North Korea's offers of dialogue are not sincere, South Korean President Lee Myung-Bak said Friday, urging the communist state to apologise for two deadly border incidents last year.

"They (North Korea) need to express their apology for what they have done," Lee told a press conference. "After that, we can move on to the next step.

"But if they threaten, attack and kill and after a period of time, say 'We should meet and talk', I think it is not sincere."

Cross-border relations have been icy since the South accused the North of torpedoing a warship in March 2010 with the loss of 46 lives.

Tensions rose further after the North's shelling of the South's Yeonpyeong border island last November, which killed four people including civilians.

The South demands its neighbour accept the blame for both incidents before relations can improve.

The North denies involvement in the warship's sinking, and says its shelling was provoked by a South Korean military drill. On Thursday it told Seoul's leaders they face a choice between dialogue or war.