'It has not worked out': Trump tweet hints that the US might take action on North Korea

"While I greatly appreciate the efforts of President Xi & China to help with North Korea, it has not worked out," he tweeted. "At least I know China tried!"

The president made the comments a day after an American student, Otto Warmbier, died in the US after being released from North Korean custody.

The specific policy implications of Trump's tweet were lost on senior administration officials who told CNN not to read too much into it, and that they did not know what he meant by it.

But the news comes as military commanders have alerted Trump that spy satellites have spotted activity around North Korea's nuclear test site, according to CNN's Barbara Starr.

Previously, 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 has repeatedly suggested his relationship with China and its president, Xi Jinping, is a tool to rein in North Korea, but he has also made clear the US would seek to unilaterally deal with North Korea should Beijing's efforts fall short.

"If China is not going to solve North Korea, we will," Trump told the Financial Times in April. "That is all I am telling you."

China has taken some measures to curb trade with North Korea by rejecting coal shipments, but it remains the North's main trade partner, engaging with it on about 90% of its outside trade. China alone could bring the collapse of the Kim regime, but that would run contrary to its interests because it would release a flood of refugees and could bring forth a democratic, united Korea in which the US could base troops.

More than presidents before him, Trump has stressed the US would take an "all options" approach to dealing with North Korea and iterated that military action could be in store.

Although experts contacted by Business Insider said prospects for diplomatic solutions with North Korea looked dim, South Korea would resolutely oppose military engagement.

Trump has also said he would be "honored" to hold talks with Kim Jong Un, but White House press secretary Sean Spicer said Tuesday that the administration was moving away from that possibility.

North Korea has multiple times floated the idea of curbing its nuclear-weapons program if the US were to scale back or stop its military exercises with South Korea. US orthodoxy on the subject maintains that the planned, regularly occurring military drills do not pose a threat to North Korea or compare with the Kim regime's regular nuclear threats, but it's possible Trump could revisit that position.

'It has not worked out': Trump tweet hints that the US might take action on North Korea

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