Recent satellite photos taken at North Korea's nuclear test site in Punggye-ri suggest that the Pyongyang regime is building a new tunnel there, raising fears that the country is preparing for a fourth nuclear test, the first since 2013.

The images of the site, which were taken between April and November, indicate that another nuclear test may be coming in the near future, according to 38 North, a website run by the U.S.-Korea Institute at the Johns Hopkins School of Advanced International Studies. The photos also identified a new tunnel entrance, the fourth at the site, and signs of construction.

Jeffrey Lewis, a nonproliferation expert at the Middlebury Institute of International Studies at Monterey, stated that despite there being no signs that a test at the site is imminent, the construction does lead to only one conclusion, adding that the new tunnel makes it "more likely that they will conduct a test in the coming year."

Since 2006, North Korea has conducted three nuclear detonations - the second in 2009 and the most recent in February 2013, reported The Washington Post. In contrast, the last time talks aimed at ridding North Korea of its nuclear weapons was in 2008, meaning two tests have taken place since then.

While it's feared that North Korea is pursuing nuclear-armed missiles designed to be able to hit the U.S. mainland, Pyongyang's missile program is particularly worrying to their neighbors - South Korea, who has repeatedly urged North Korea to abandon the program if it hopes to have any dialogue with them. 

"It will be possible only when the North comes forward for a proactive and sincere dialogue. What counts most is North Korea's sincerity and determination to act on its words," South Korea President Park Geun-hye said in November.

The revelation of images of the ongoing construction at the mountainous Punggye-ri site comes after Korea's Yonhap news agency quoted a South Korean government source in October, who said that workers were constructing a tunnel there.

The photos' release also comes after North Korea reported in September that the reactor at its Yongbyon nuclear facility has resumed normal operations after shutting it down in 2007.