Japan and the European Union condemned North Korea's reported nuclear test as a violation of U.N. sanctions, as several leaders, including President Barack Obama, suggested Monday that they would bring the matter to the Security Council.
The test - North Korea's second since 2006 - drew near universal disapproval, although China had no immediate response.
"North Korea is directly and recklessly challenging the international community," Obama said in a statement. "North Korea's behavior increases tensions and undermines stability in Northeast Asia."
Obama said Washington will be working in the days ahead with the Security Council and other nations that have been trying to persuade North Korea to give up its nuclear ambitions.
Japanese Prime Minister Taro Aso reacted even more sternly, calling the test a violation of UN sanctions.
"It is unacceptable," Aso said, adding that an emergency meeting of the Security Council expected later Monday in New York would deal with the test, North Korea's second since 2006. Japan is particularly sensitive to such actions because its territory is within North Korean missile range.
UN Secretary-General Ban Ki-moon said he was deeply worried by a nuclear test by North Korea, according to Danish TV2.
"I am deeply worried by the reports of a nuclear test in North Korea. I closely follow the situation in the region - just like I will follow an emergency session in the U.N. Security Council later today," TV2 quoted Ban Ki-moon as saying in an interview.
In Brussels, the European Union's foreign policy chief, Javier Solana, denounced the test as a flagrant violation of Security Council resolutions.
"These irresponsible acts by North Korea warrant a firm response by the international community," Solana said in a statement. "The European Union will be in contact with its partners to discuss appropriate measures."
French Foreign Ministry spokesman Eric Chevallier suggested the Security Council may discuss "the strengthening of sanctions."
North Korea has been under UN sanctions that bar it from nuclear and ballistic activity since its first atomic test in 2006. A rocket launch in April - that the North said put a satellite into orbit but critics called a test of long-range missile technology - drew further condemnation from the world body.
In response, the North announced that it was withdrawing from talks aimed at halting its nuclear program and said it would restore atomic facilities it had begun disabling under those negotiations.
The move has been seen as reflecting North Korea's defiance in the face of the criticism in April.
North Korea's official KCNA news agency reported Monday that a successful underground nuclear test was "safely conducted on a new higher level in terms of its explosive power and technology of its control."
Russia's Defense Ministry said it confirmed the test and estimated its yield at 10 to 20 kilotons - a size comparable to the bombs that flattened Hiroshima and Nagasaki.
Defense Ministry spokesman Alexander Drobyshevsky, speaking on state-run Rossiya television, said the test was conducted about 50 miles (80 kilometers) northwest of the city of Kilchu.
South Korea called the test a threat to world peace.
"We are seriously concerned about North Korea's second test of a nuclear device," South Korean Foreign Minister Yu Myung-hwan told the Associated Press. "It's a direct threat against the peace and stability in the region as well as to the world."
Swedish Foreign Minister Carl Bildt, speaking in Hanoi where EU and Southeast Asian leaders are to meet, said the announcement appeared to be true and called it "alarming."
Australian Foreign Minister Stephen Smith told Parliament that final confirmation may take days, but said Australia is proceeding as if the reports are true and called the test a "flagrant breach of its international obligations" and a violation of UN sanctions that should be condemned.
Nuclear-capable India was also strongly against the North's test.
"India is against nuclear proliferation," India's Defense Minister AK Antony said. "It's a matter of very serious concern not only to India, but to the world."Reuse content