(Yonhap) -- North Korea is believed to be gearing up for a nuclear test, an intelligence official said Sunday, a move certain to fuel the already high tensions over its planned long-range rocket launch.

   Satellite images show the communist nation digging a new tunnel underground in the Punggye-ri nuclear test site in the country's northeast, where it conducted two previous nuclear tests, first in 2006 and then in 2009.

   The construction is believed to be in its final stage, the official said.

   "North Korea is making clandestine preparations for a third nuclear test at Punggye-ri in North Hamkyong Province, where it conducted two nuclear tests in the past," the official said on condition of anonymity.

   Commercial satellite imagery showed piles of earth and sand at the entrance of a tunnel in the Punggye-ri site. The soil is believed to have been brought to the site to plug the tunnel, one of final steps before carrying out a nuclear test blast.

   A nuclear test following a long-range missile test fits the pattern of North Korean behavior.

   In 2006, the provocative regime carried out its first-ever nuclear test, three months after the test-firing of its long-range Taepodong-2 rocket. The second nuclear test in 2009 came just one month after a long-range rocket launch.

   The North says it will fire off its Unha-3 long-range rocket between April 12-16 to put what it claims is a satellite into orbit. But regional powers believe the launch of Kwangmyongsong-3 is a pretext to disguise a ballistic missile test banned under a U.N. Security Council resolution.

   A government source in Seoul said Sunday that the North has already set up the first, second and third-stage boosters and the payload on a launch pad at the Tongchang-ri missile base in the North's northwest.

   "It has been confirmed that even the payload was installed in addition to the first, second and third-stage boosters. At present, the joint parts are being examined," said the source, forecasting that the injection of liquid fuel is expected to begin on Monday with the launch likely between April 12 and 13.

   The North's official Korean Central News Agency said in a dispatch that reporters from more than 20 media firms, including the Associated Press, CNN, Reuters, AFP, BBC, Kyodo News and NHK, arrived in Pyongyang by Saturday to observe the launch of

Kwangmyongsong-3. According to some wire reports, the foreign reporters were brought to the Tongchang-ri facility over the weekend to see the Unha-3 and Kwangmyongsong-3.

   Pyongyang has claimed that it has a sovereign right to fire the rocket for the peaceful exploration of space, though the launch is widely seen as a disguised test of international ballistic missile technology banned under a U.N. resolution.

   But the top diplomats from South Korea, China and Japan agreed that North Korea's planned long-range rocket launch is a violation of a U.N. Security Council resolution. South Korean Foreign Minister Kim Sung-hwan, Chinese Foreign Minister Yang Jiechi and Japanese Foreign Minister Koichiro Gemba shared the understanding during annual three-way talks and bilateral meetings on Saturday and Sunday in the eastern Chinese city of Ningbo.

   The North's nuclear and missile programs have long been a regional security concern. The country is believed to have advanced ballistic missile technology, though it is still not clear whether it has mastered the technology to put a nuclear warhead on a missile.