A fortified installation, believed to be a "command bunker," has been found at North Korea's nuclear test site where the country may likely conduct another nuclear test soon, a U.S. think tank said Friday.

According to "38 North", an analysis program of the U.S.-Korea Institute at Johns Hopkins University in Maryland, the "command bunker" is located about 150 meters from the entrance to a tunnel that may be used to carry out the nuclear test.

The Punggye-ri test site on the North's northeastern coastal region is where the country conducted its first and second underground nuclear tests in 2006 and 2009.

Based on its analysis of satellite imagery, the institute said, the fortified installation is believed to have been built to house monitoring and other communications equipment and protect engineers and other personnel involved in nuclear tests.

Also sighted near the bunker is a radio relay station apparently for quick communications contact with the leadership in Pyongyang, it said.

Judging by the amount of digging and excavation work, the reinforced facility may be about 92.9 square meters, the institute said, adding that construction might have begun after 2005.

It said photos taken in December showed a well-maintained road west of the bunker.

South Korean experts said that if there is a lot of activities around the bunker, this can be an indication that the North is close to conducting its third nuclear test.

Following a punitive resolution adopted by the U.N. Security Council on Wednesday, North Korea issued a string of warnings that it would build up its nuclear deterrence that would mainly target the United States.

(Yonhap News)