UN leader Ban Ki-moon warned Thursday that a nuclear test by North Korea could blow up hopes of an eventual reconciliation by "tying the hands" of the South's incoming president.
Ban, a former South Korean foreign minister, said incoming South Korean president Park Geun-Hye is "very much committed" to improving relations with North Korea.
"If they conduct this nuclear test, it may be the case that they are effectively tying the hands of the new president of Korea," Ban told a small group of reporters, including AFP.
"It may take a long time before any initiative between North and South can take place to normalize this relationship," he said, adding to international warnings to the isolated North.
Park will take over on February 25 from President Lee Myung-Bak, who warned on Thursday of "serious consequences" if Pyongyang stages the test.
The two sides have been divided since the end of the 1950-53 Korean war, and the 2010 sinking of a South Korean warship and a subsequent missile attack further escalated the rivalry.
A third test of an atomic weapon would be going in "the wrong direction" said Ban, highlighting UN resolutions that imposed tough sanctions after blasts in 2006 and 2009.
The UN Security Council has already threatened "significant" measures if North Korea stages a new breach of the resolutions. Ban said he has been discussing the North's moves "with key countries."
Ban said the Stalinist government should do more to help its people. "The humanitarian situation is dire in DPRK," Ban said, using the acronym of the North's official name, the Democratic People's Republic of Korea.
He said nations had not given money to UN humanitarian efforts in North Korea "because of this crisis and the very tense situation on the Korean peninsula."
Ban said he had been forced to use money from the UN's emergency fund to support relief efforts in North Korea, where there is again widespread hunger.