North Korea tests nuclear weapon
In a statement released by his official spokesman, Annan described the test as a threat to regional stability and a violation of international agreements, AFP reported.
"The Secretary General is deeply concerned by the news that the Government of the Democratic People's Republic of Korea has conducted a nuclear test, in spite of repeated worldwide efforts to discourage it from doing so," the statement said.
The UN Security Council has strongly condemned the test.
President George W. Bush said the U.S. is working to confirm the test claim. He branded it a "provocative" act threatening peace and stability.
He said he and regional leaders agreed North Korea's actions were unacceptable and deserved an immediate UN response.
Secretary of State Condoleezza Rice spoke by telephone Monday with her counterparts from China, Russia, Japan and South Korea, her spokesman said. Rice also talked to foreign ministers Alexander Downer of Australia and Winston Peters of New Zealand.
Japan's UN ambassador, Kenzo Oshima, urged North Korea to refrain from further testing and to return to six-party talks.
He said the Security Council would now begin work on a resolution being drafted by the United States.
The session comes three days after the council agreed a formal statement urging North Korea to cancel any planned nuclear test and return to disarmament talks.
UK Foreign Secretary Margaret Beckett said the UK would be "pushing for a robust response" under Chapter Seven.
"Put simply, this means we shall be pushing for sanctions against North Korea," she said.
U.S. ambassador to the UN John Bolton said that although responses were preliminary, he was "very impressed by the unanimity of the Council on the need for a strong and swift answer".
"I didn't see any protectors of North Korea in that room," he said.
But North Korea's ambassador, Pak Gil Yon, said the Security Council should congratulate Pyongyang instead of issuing "useless" resolutions. In his first public statement, the U.S. president said the North Korean action "constitutes a threat to international peace and security."
He said he had telephoned Chinese, Japanese, Russian and South Korean leaders, who had all reaffirmed their commitment to a nuclear-free Korean peninsula.
Earlier Japan's Prime Minister Shinzo Abe - visiting Seoul - called the test "unpardonable".
He warned the region was "entering a new, dangerous nuclear age".
South Korean President Roh Moo-hyun feared the move could "spark a nuclear arms build-up in other countries".
In an unusually strong statement against its ally, China said the test "defied the universal opposition of international society".
Meanwhile, the head of the South's intelligence service said it had detected more movement at another North Korean test site and he could not rule out further nuclear tests.
'No radiation leak'
South Korean media said the test took place in Gilju in Hamgyong province at 1036 (0136 GMT).
The size of the bomb is uncertain, with estimates varying from 550 tons of destructive power to as much as 15 kilotons. The 1945 Hiroshima bomb was 12.5-15 kilotons.
U.S. intelligence has detected an explosion of less than one kiloton in magnitude in North Korea but has not been able to determine whether it was nuclear or not, AFP quoted a senior intelligence official as saying Monday.
North Korea's KCNA news agency described the test as an "historic event that brought happiness to our military and people".
It said the test would maintain "peace and stability" in the region and was "a great leap forward in the building of a great prosperous, powerful socialist nation". There was no radiation leak, it said.
Pyongyang pulled out of the nuclear Non-Proliferation Treaty in 2003 and has refused for a year to attend talks aimed at ending its nuclear ambitions.