Pakistan has criticised North Korea's nuclear test, saying it was a "destabilising" development for the entire region.
There was concern among South Koreans at the news
Pakistan's disgraced nuclear scientist AQ Khan admitted in 2004 that he had passed on nuclear secrets to North Korea, Libya and Iran.
India has also expressed deep concern about the test and said it "jeopardises" peace and stability.
India and Pakistan carried out nuclear tests in 1998.
"We had urged [North Korea] to desist from introducing nuclear weapons in the Korean peninsula," Pakistani foreign office spokeswoman Tasnim Aslam said.
"It is regrettable that [North Korea] chose to ignore the advice by the international community not to conduct the test."
Ms Aslam has also defended Pakistan's own nuclear record.
"Pakistan did not initiate nuclear tests in the region. We were acting purely in self-defence," she said.
She said that unlike South Asia in 1998, "the Korean peninsula was a nuclear-free zone," she said.
"It is going to have a chain reaction that nobody wants. We are talking about it because of its ramifications on international peace and security."
India said the "test also highlights the dangers of clandestine proliferation".
North Korea says the underground test was a success and had not resulted in any leak of radiation.
The White House said South Korean and US intelligence had detected a seismic event at a suspected test site.
The White House said, if confirmed, the test would be a "provocative act", while China denounced it as "brazen".
Japan's Prime Minister Shinzo Abe said the test was "unpardonable".
South Korea said it would "sternly respond".
Seoul also suspended a scheduled aid shipment to North Korea, the state news agency reported.
The US Geological Survey said it had detected a 4.2 magnitude quake in North Korea, while a South Korean official said a 3.5 magnitude seismic tremor had been detected in north Hamgyong province, in the north-east.
KOREAN NUCLEAR CRISIS
Sept 2005: At first hailed as a breakthrough, North Korea agrees to give up nuclear activities
Next day, N Korea says it will not scrap its activities unless it gets a civilian nuclear reactor
US imposes financial sanctions on N Korea businesses
July 2006: N Korea test-fires seven missiles
UN Security Council votes to impose sanctions over the tests
Oct 2006: N Korea threatens nuclear test
South Korea's Yonhap news agency is reporting that the test took place in Gilju in Hamgyong province at 1036 (0136 GMT).
A top Russian military officer said it was "100%" certain that an underground nuclear explosion had taken place.
When it announced the test, KCNA described it as an "historic event that brought happiness to our military and people".
"The nuclear test will contribute to maintaining peace and stability in the Korean peninsula and surrounding region," KCNA said.
The region has been on high alert since North Korea announced last week that it would conduct a nuclear test.
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