Story highlights The activity appears to involve some modifications around one of the tunnel entrances to an underground test area

US officials have known that the site is ready to conduct an underground test for some time

Washington (CNN) US spy satellites have detected new activity at North Korea's underground nuclear test site for the first time in several weeks, two US officials told CNN.

The activity appears to involve some modifications around one of the tunnel entrances to an underground test area.

The officials 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.

US officials have known that the site is ready to conduct an underground test for some time.

Two senior US officials with direct knowledge also told CNN 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.

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