South Korea's outgoing President Lee Myung Bak said the North must chose between either seeking nuclear weapons and staying isolated or abandoning them and joining the world community, DPA reported.
"North Korea's pursuit of nuclear weapons and missiles is making the lives of North Korean citizens more difficult," Lee said in an interview with dpa and other news organisations.
He was speaking before Pyongyang on Saturday confirmed plans to launch a rocket this month. The North says it is sending a satellite into orbit but much of the world sees it as a disguised missile test.
The South Korean president, whose five-year term ends in late February, confirmed media reports that the North was planning the launch and said this would violate UN Security Council resolutions.
He accused the North of seeking to influence a December 19 presidential vote in the South but said this was bound to fail.
Lee stressed that the North faces a choice: "Will it keep going through hard times with its nuclear arsenal or abandon it and choose the path to prosperity by joining the international community?"
The president said that "it is obvious that, unlike before, time is no longer on North Korea's side."
Even its ally Beijing wants Pyongyang to concentrate on improving the lives of the country's people rather than on its weapons programme, he said.
Lee urged Pyongyang to change course and offered Seoul's help if it decided to do so.
The leadership change from Kim Jong Il, who died almost a year ago, to his son Kim Jong Un provided such an opportunity, he said, cautioning however that it was too early to judge the new leader.
Despite the tensions that often flare between the North and South, Lee said he believes reunification is inevitable and pointed out that "German reunification came more quickly than expected."
He said reuniting the Koreas would be more difficult because of the larger wealth gap but stressed: "The important thing is that reunification could come at anytime. I firmly believe in that."