President Roh Moo-hyun has urged South Koreans to remain calm, despite the North's withdrawal from nuclear talks.
On the second anniversary of his inauguration, Mr Roh said a flexible approach was needed to convince the North to end its nuclear programmes.
It was the first time Mr Roh had spoken about Pyongyang's recent announcement that it would not attend nuclear talks and that it possessed nuclear weapons.
Mr Roh also said Seoul's links with Washington were more stable than ever.
"My beloved citizens. The North Korean nuclear issue must give you cause for great concern," President Roh said, in the televised address.
"Although an unexpected development occurred, it doesn't greatly change the fundamental structure" of the nuclear standoff, he said.
"We will be flexible, but won't lose our principled stance."
He was referring to an announcement by North Korea's foreign ministry on 10 February, which said "there is no justification for us to participate in the six-party talks again, given that the Bush administration termed [us] an outpost of tyranny".
Senior negotiators from Washington and Tokyo are due to arrive in Seoul on Saturday to attempt to revive the stalled talks - which would involve North and South Korea, Japan, China, the US and Russia.
The last time six-nation discussions were held - in Beijing last June - the meeting ended without agreement.
A subsequent round of talks planned for September never took place, as Pyongyang refused to turn up, citing US "hostility".
Despite the strongly-worded statement on 10 February, prospects for the talks looked brighter after North Korean leader Kim Jong-il indicated on Tuesday that he might return to the negotiating table if the conditions were right.
The nuclear standoff erupted in October 2002 when the US accused North Korea of having a secret uranium-enriching programme carried out in defiance of a 1994 agreement.
Pyongyang denied that charge but has since restarted its plutonium programme.