South Korean officials have detected "brisk" activity at a North Korean nuclear test site, a news report said Thursday, days after the communist country threatened to conduct nuclear and missile tests.

Last week, the North said it would carry out a second nuclear test and test-launch intercontinental ballistic missiles, unless the U.N. Security Council apologizes for criticizing the country's April 5 rocket launch.

On Thursday, South Korea's mass-circulation Chosun Ilbo newspaper reported that the South's authorities have continuously detected "brisk" activities of personnel and vehicle movements at the North's nuclear site in the northeastern county of Kilju, where the North conducted its first-ever nuclear test in 2006.

The paper quoted an unnamed South Korean government source as saying that the North is believed to be preparing to conduct a nuclear test soon. The paper didn't say how South Korea obtained the intelligence.

South Korea's Defense Ministry, Foreign Ministry and the National Intelligence Service — the country's main spy agency — said they cannot confirm the report.

The newspaper also said North Korea is speeding up construction of a new west coast missile test site by recently deploying more workers and equipment there. The paper said the South Korean military believes the North may implement its threatened long-range missile tests at the new site.

The paper quoted the source as saying the North is expected to advance the construction of the new launch site by several months, initially set for the end of this year.

In November, South Korea's defense minister told parliament that construction of the North's new missile site began eight years ago and is about 80 percent complete. South Korean officials have said the new site appears designed to launch larger missiles or satellite projectiles than the North's present east coast Musudan-ni site.

Tension on the Korean peninsula has spiked since the North pressed ahead with its rocket test in defiance of international warnings. Pyongyang said it put a satellite into orbit as part of its peaceful space development program, but the U.S. and its allies have said the blast was a cover to test the country's advanced missile technology.

The U.N. Security Council denounced the launch and imposed sanctions on North Korean companies, prompting Pyongyang to boycott six-nation disarmament talks on its nuclear programs and threaten to conduct nuclear and missile tests.

In Beijing, meanwhile, American nuclear negotiators were hoping to devise a strategy for reviving the stalled talks.

Special envoy Stephen Bosworth and Ambassador Sung Kim were scheduled to arrive in Beijing in the afternoon to meet with their Chinese counterparts.

Restarting the talks poses a complex challenge for President Barack Obama's new administration and has taken on new urgency since the North said it was restarting its atomic program.

The U.S. State Department said its two envoys will travel to Seoul on Friday before flying to Tokyo and Moscow next week — a comprehensive tour of all the countries involved in the negotiations. The diplomats have no plans to travel to North Korea.