Recent satellite images indicate that North Korea is building a new tunnel at its nuclear test site, a respected U.S. think tank has said, according to BBC News.

A report on 38 North, a website run by the U.S.-Korea Institute at the John Hopkins School of Advanced International Studies, studied images taken between April and November.

These appeared to show work in a new area of the Punggye-ri nuclear zone.

But, the report said, there is no sign that any nuclear test is imminent.

North Korea carried out three underground nuclear tests at Punggye-ri in 2006, 2009 and 2013. Its nuclear programme has been a source of great concern for the international community.