"This should be a moment of joy. But instead, I stand here with a very heavy heart," Ban Ki-Moon told reporters in Seoul, South Korea. "Despite the concerted warning from the international community, North Korea has gone ahead with a nuclear test."
Ban, 62, was nominated by the U.N. Security Council to succeed Kofi Annan, whose term expires at the end of the year. He faces likely confirmation by the U.N. General Assembly.
Japan's U.N. Ambassador Kenzo Oshima asked the 192-nation world body to act promptly to give final approval to Ban so he can have a sufficient transition before taking over as U.N. chief on Jan. 1, after Annan's second five-year term ends.
"I think the fact that the candidate is currently foreign minister of the Republic of Korea is an asset in dealing with the situation in the Korean peninsula that we are now facing," he said.
U.S. Ambassador John Bolton called Ban's selection "a very significant event," saying the United States looks forward to quick approval by the General Assembly.
"It's really quite an appropriate juxtaposition that today 61 years after the temporary division of the Korean peninsula at the end of World War II, we're electing the foreign minister of South Korea as secretary-general of this organization and meeting as well to consider the testing by the North Koreans of a nuclear device," he said.
"I can't think of a better way to show the difference in the progress of those two countries — great progress in the south and great tragedy in the north," Bolton said.
In Seoul, Ban told reporters that if the General Assembly appoints him, he would "contribute as much as I can to the resolution of all kinds of problems including the North Korean nuclear issue that may threaten international peace and security."
Ban called North Korea's reported nuclear test an "act of provocation" and "a grave and direct threat to the peace and stability on the Korean Peninsula and in Northeast Asia."
South Korea will "be firm and resolute in adhering to the principle of no tolerance of a nuclear North Korea," he added.
North Korea has not mentioned Ban's bid to become U.N. secretary-general. But it previously has accused him of blindly following the U.S. line by urging the North to resume negotiations and give up the atomic weapons program.
The reported test drew immediate condemnation from members of the U.N. Security Council, with the U.S. saying it would seek U.N. sanctions to curb North Korea's import and export of material for weapons of mass destruction, as well as its illicit financial activities.
Security Council members also demanded at an emergency meeting that the communist nation return to six-party talks on its weapons program, Japan's U.N. ambassador said.
The latest incident has added new urgency to international efforts to solve the North's nuclear weapons ambitions. The six-party talks have been stalled for almost a year and no progress has been made on implementing a September 2005 agreement in which North Korea pledged to give up its nuclear programs in exchange for aid and security guarantees.
"In close cooperation and consultation with the countries concerned and the international community, we will seek firm and strong measures so as to get North Korea to abandon all of its nuclear weapons and related programs," Ban said.
Ban has been South Korea's foreign minister for more than 2½ years and served as national security adviser to two presidents — jobs that focused on relations with the North. He has served as a diplomat for nearly 40 years, including previous stints at the U.N. and in Washington.
Ban will be the eighth secretary-general in the United Nations' 60-year history, overseeing an organization with some 92,000 peacekeepers around the world and a $2 billion annual operating budget. Fighting hunger, assisting refugees and slowing the spread of HIV/AIDS are all programs that fall under the secretary-general's purview.