North Korean leader Kim Jong Un meets scientists and technicians in the field of researches into nuclear weapons in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang. Reuters/KCNA President Donald Trump tweeted on Tuesday that China's effort to reel in North Korea "has not worked," leaving officials who commented to CNN baffled as to his meaning — but reports have started to circulate indicating that North Korea may be planning to test a nuclear device.

Spy satellites over spotted activity at North Korea's nuclear test site that could indicate preparations for a possible detonation, and Trump has been briefed on his options, should he choose to act, according to CNN.

Additionally, the online flight monitor CivMilAir has detected a flight of the WC-135, or the Constant Phoenix "nuke sniffer," which surveys the atmosphere to detect nuclear explosions. While CivMilAir has not pinpointed the location of the WC-135, it regularly flies around North Korea.

In April, Trump told CBS he would "not be happy" if North Korea tested a nuclear device, though he did not specify whether he would take action in response. Trump did not specify if "not being happy" meant that military action would follow.

At that time there was a flurry of speculation the North Koreans may test another nuclear device, but they have not since then. While North Korea has carried out missile tests almost weekly since April, it has only carried out six nuclear tests.

Generally, nuclear tests draw harsher condemnation from the United Nations, with China, North Korea's sole ally, signing on to increased sanctions on North Korea every time after a nuclear detonation.

Experts have told Business Insider that a military intervention against North Korea remains unlikely because of the spread and mysterious nature of the country's arsenal, but a limited strike against a single, high-value target could happen.

North Korea, the only country to test nuclear weapons in the 21st century, carries out its nuclear tests in a hardened structure underground.