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.
The West is concerned that North Korea may be on a path to developing an intercontinental ballistic missile (ICBM).
North Korea has declared that it could test-launch an ICBM “at any time” from any location set by its leader, Kim Jong Un, though it remains unclear whether any of its recent tests have been of an ICBM.
U.S. officials told CNN Tuesday that if a sixth nuclear test by North Korea were to occur, it would be clear that the existing pressure by China on North Korea is not working. However, other senior officials said the administration's policy hinges on pressuring North Korea through China.
Trump has often cited China, North Korea's longtime ally, as a key player in U.S. efforts to rein in North Korea's quest to develop long-range nuclear missiles.
On Tuesday, however, he took to Twitter to offer a grim assessment of China's role in restraining North Korea, writing, "While I greatly appreciate the efforts of President Xi & China to help with North Korea, it has not worked out. At least I know China tried!"
Multiple U.S. officials told CNN they are checking on what the President meant in his latest tweet about how China's efforts in North Korea have "not worked out."
White House press secretary Sean Spicer painted a rosier picture on Tuesday, telling reporters that the U.S. is continuing to work with China to apply pressure on North Korea and said the U.S. has seen "positive movement" with China.