China, Russia and the United States have all built new facilities and dug new tunnels at their nuclear test sites in the last few years, reports CNN, citing satellite images that show the new construction and increased vehicle traffic coming in and out of the sites.
The images were "exclusively" obtained and provided by a prominent analyst in military nonproliferation studies, according to the CNN report Friday.
"There are really a lot of hints that we're seeing that suggest Russia, China and the United States might resume nuclear testing," the news outlet quoted Jeffrey Lewis, an adjunct professor at the James Martin Center for Nonproliferation Studies at the Middlebury Institute of International Studies.
None of the countries have conducted nuclear tests since they were banned by the 1996 Comprehensive Nuclear Test Ban Treaty.
Lewis said the Russian military's poor performance in Ukraine may have prompted Russia to consider a resumption of nuclear testing.
CNN notes the activity takes place "at a time when tensions between the three major nuclear powers have risen to their highest in decades."