North Korea has intensified activities at weapons sites after threatening to stage more nuclear and missile tests in response to UN sanctions, a South Korean newspaper reported Thursday.

The Chosun Ilbo said busy movements of vehicles and people had been spotted in the northeastern county of Kilju, where the communist state carried out its first atomic test in October 2006.

The North has also been speeding up construction of a new long-range missile launch site on the west coast, the largest-circulation daily said, citing a government source.

South Korea's National Intelligence Service declined comment on the report.

"It's hard to forecast the timing for an underground nuclear test but the North is believed to be ready to do so at short notice," the source was quoted as telling the daily.

The North has also brought more people and equipment to the new launch site at Dongchang-ri, the report said.

The site 120 kilometres (75 miles) northwest of Pyongyang was initially expected to be finished by year-end but is now likely to be completed a few months earlier than anticipated, the source said.

South Korea's defence minister said last November the site was 80 percent complete and would be able to handle larger missiles than those previously fired.

The North has a separate site at Musudan-ri on the east coast, where it launched a long-range rocket on April 5. It said it put a peaceful satellite into orbit but other nations saw the launch as a disguised missile test.

After the UN Security Council condemned the launch and tightened sanctions, the North quit six-party nuclear disarmament talks and said it restarted a programme to make weapons-grade plutonium.

Last week it announced it would conduct a second nuclear test and ballistic missile tests unless the United Nations apologises for condemning and punishing its rocket launch.

Stephen Bosworth, the US special envoy for North Korea, was due Thursday in China on the first leg of a tour which will also take him to South Korea, Japan and Russia. He has no current plans to visit North Korea.

Bosworth will consult the fellow six-party members on ways to persuade the North to come back to negotiations.

Even before Pyongyang's pullout, the long-running talks were deadlocked by a dispute over how to verify the North's declared nuclear activities.

US Secretary of State Hillary Clinton has played down hopes of an early breakthrough, saying last week that "at this point (it) seems implausible if not impossible" that the North will return to the six-party talks.

In Washington President Barack Obama spoke by phone Wednesday with his Chinese counterpart Hu Jintao and shared his "concerns" over security issues including North Korea's nuclear programme, the White House said.