In a move that would cross a red line for the United States and its allies, North Korea appears to be making preparations at its main nuclear test site that would allow it to quickly conduct its seventh atomic test, a media report quoting South Korean government sources said Sunday.

Pyongyang appears to be working on a "shortcut" to a tunnel at its Punggye-ri underground test facility, apparently in an attempt to repair the site, which was shuttered in 2018, the Yonhap news agency reported, quoting unidentified sources in Seoul.

South Korea's military and intelligence authorities detected signs that the North was focusing on restoring Tunnel 3 in the mountainous northeastern area, the sources said.