South Korean and American intelligence authorities have detected signs of massive excavation at North Korea’s nuclear test site.

A South Korean source said on Friday that North Korea is excavating tunnel 4 at its Punggye-ri nuclear test site in North Hamgyong Province, mobilizing a number of coal mine vehicles.

The source said that it’s unlikely the North would carry out a nuclear test when excavation work is under way nearby, meaning an expected nuclear test will likely be delayed.

There are speculations that growing pressure from the U.S. and China might have encouraged the North to postpone a planned test. China recently beefed up its pressure against Pyongyang by threatening to slash its oil supplies to the North. 
 
South Korea and the U.S. have closely monitored the North’s movement on the assumption that the tunnels at the Punggye-ri site are ready and that the North can conduct a nuclear test any time.