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 is a pretext to disguise a ballistic missile test banned under a U.N. Security Council resolution.

Sources said the North is believed to have put the rocket on a launch pad in the country's northwest on Friday.

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. (Yonhap News)