U.S. spy satellites have detected new activity at North Korea's underground nuclear test site for the first time in several weeks, two American officials told CNN on Tuesday.
The activity appears to involve some modifications around one of the tunnel entrances to an underground test area, according to the officials.
They said it is not yet clear if the activity indicates a sixth nuclear test is imminent, but noted there is concern that North Korea could set off a test during Wednesday's visit to Washington by top Chinese diplomats and military officials.
U.S. officials have known that the site is ready to conduct an underground test for some time, according to CNN.
Two senior officials with direct knowledge also told the network that military options for North Korea have recently been updated, and will be presented to President Donald Trump for a decision to act if there is a nuclear test.
North Korea has continuously carried out nuclear tests and ballistic missile tests in violation of UN resolutions.