The U.N. chief said Monday that a second nuclear test by North Korea would represent "a clear violation" of a United Nations Security Council resolution.
The 15-nation council has called an emergency session on the matter for later Monday at U.N. headquarters in New York.
"I sincerely hope that the Security Council will take necessary corresponding measures," U.N. Secretary-General Ban Ki-moon told The Associated Press, declining to specify what further measures, or sanctions, he would urge the council members to take.
Ban, who was in Copenhagen for a global business summit on climate change, said he would closely monitor the meeting in New York.
New testing by North Korea would undermine peace and security in the region, Ban said, and he urged the communist nation "to refrain from taking any action which will deteriorate the situation."
"I'm deeply disturbed by a report of nuclear testing," he said. "If it is confirmed, it is a clear violation of Security Council resolution 1718 of 2006. And it runs counter to the ongoing international community's efforts to curb nuclear proliferations."
Ban also said the announcement from the nation's official news agency that it carried out an underground nuclear test Monday far bigger than one conducted three years ago "will create negative impact to ongoing negotiation on nuclear disarmament."
"They should have come to the dialogue table and resolved all the issues through peaceful means," he said. Nuclear testing by North Korea "creates tensions in the region and it creates negative impact to regional peace and security."
An underground nuclear test, along with a series of test-firings of ground-to-air missiles claimed by North Korea on Monday swiftly drew international condemnation.
European Union Commission President Jose Manuel Barroso said he "strongly condemns that provocation from the North Korea regime."
"I hope the international community will be very clear in its reaction. I also encourage the North to refrain from all kinds of provocation," Barroso said at a news conference.