South Korea and the United States will hold a joint naval exercise beginning tomorrow involving a US nuclear submarine, in a move seen as a warning to North Korea ahead of its widely expected nuclear test.
The three-day exercise will be held in the Sea of Japan, or East Sea, off the South Korean port city of Pohang, the Yonhap news agency reported.
Meanwhile North Korea accused the US of double standards and said it would face the "toughest retaliation" for criticising its rocket launch while supporting one by South Korea.
The UN Security Council has imposed new sanctions on Pyongyang for its launch, conducted last month.
Analysts say Pyongyang has resumed work for a third nuclear test a week after it threatened to conduct one in response to strengthened UN sanctions.
The North said the rocket launch was a scientific mission aimed at placing a satellite in orbit, but most of the world saw it as a disguised ballistic missile test.
South Korea's Joint Chiefs of Staff Jung Seung-jo said on Friday that the drill aimed to test combat readiness between Seoul and Washington while guarding against possible North Korean provocations involving submarines, Yonhap reported.
The 6,900-tonne US nuclear submarine San Francisco and the 9,800-tonne Aegis destroyer Shiloh were being mobilised for the exercise.
"The presence of a US nuclear submarine here would itself serve as a message to North Korea," Jung said.
The North has reportedly covered the entrance to a tunnel at its nuclear test site in an apparent effort to avoid satellite monitoring of its preparations for a possibly imminent detonation. A camouflage net was placed on the tunnel entrance at Punggye-ri in the northeast of the country, the site of the two previous nuclear tests in 2006 and 2009.
A government source in Seoul said increased activity had been spotted at the site, which has three tunnel entrances and multiple support buildings. "At a tunnel in the southern part of the test site in Punggye-ri, we've found that work presumed to be part of preparations for a nuclear test has entered its final stage."
In Washington, former American ambassador to the UN Bill Richardson urged the US to engage in dialogue with North Korea, but only if Pyongyang refrained from more nuclear tests and missile launches.
Last month, Richardson and Google chairman Eric Schmidt visited North Korea in an unsuccessful attempt to free Kenneth Bae, an American of Korean descent who is being held in the country.
Richardson later concluded that a lack of direct dialogue between the US and North Korea was not helping Washington achieve its goals.
"Dialogue is not an endorsement or legitimisation of your counterpart's positions," the former diplomat wrote in The Washington Post. "Rather, it is an exchange of arguments and ideas that help both sides better understand the other and identify opportunities."
He believes the US cannot rely on China to prevent nuclear proliferation. Issues such as zones of influence and refugees complicate the Sino-North Korean relationship. Recent public statements from both Seoul and Pyongyang hinted at a renewed interest in dialogue, he said.
While he approved of recently toughened UN sanctions against Pyongyang in response to its ballistic missile launch, Richardson also argued for keeping all diplomatic options open. "While sanctions are merited and are a legitimate tool, so is dialogue. The two are not mutually exclusive."
Additional reporting by Associated Press