Russia, the United States and China have all built new facilities and dug new tunnels at their nuclear test sites in recent years, satellite images obtained exclusively by CNN show, at a time when tensions between the three major nuclear powers have risen to their highest in decades.

While there is no evidence to suggest that Russia, the US or China is preparing for an imminent nuclear test, the images, obtained and provided by a prominent analyst in military nonproliferation studies, illustrate recent expansions at three nuclear test sites compared with just a few years ago.

One is operated by China in the far western region of Xinjiang, one by Russia in an Arctic Ocean archipelago, and another in the US in the Nevada desert.

The satellite images from the past three to five years show new tunnels under mountains, new roads and storage facilities, as well as increased vehicle traffic coming in and out of the sites, said Jeffrey Lewis, an adjunct professor at the James Martin Center for Nonproliferation Studies at the Middlebury Institute of International Studies.