North Korean Missile
A ballistic rocket is test-fired through a precision control guidance system in this undated photo released by North Korea's Korean Central News Agency (KCNA) May 30, 2017. KCNA/Reuters

U.S. spy satellites picked up on activity at North Korea’s only nuclear testing site, Punggye-ri, according to CNN Tuesday night. The activity appears to be changes made to a tunnel entrance to one of the underground testing facilities.

The test site is located in the north of the country in North Hamgyong Province, close to North Korea’s border with China. The activity overshadows Secretary of State Rex Tillerson and Defense Secretary Jim Mattis’ Wednesday meeting with their Chinese counterparts.

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Also overshadowing the meeting is a mysterious tweet by President Donald Trump Tuesday criticizing the Chinese president.

“While I greatly appreciate the efforts of President Xi (Jinping) & China to help with North Korea, it has not worked out. At least I know China tried!,” Trump wrote.

The New York Times reported Wednesday that administration officials were considering imposing sanctions on Chinese businesses that have ties to North Korean an effort to help curb their nuclear program, and also that officials found the Trump tweet perplexing.

North Korea has had five nuclear test so far and U.S. military options are prepared if there’s a sixth.

“Two senior US officials with direct knowledge (said) 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,” CNN reported Tuesday.

Adding to tension between the United States and North Korea is the death of 22-year-old college student Otto Warmbier. The University of Virginia student was imprisoned for 17 months in North Korea after allegedly attempting to remove a propaganda poster. Warmbier was returned to the U.S. in a coma and with severe neurological damage. He died Monday.

When President Xi visited President Trump at Mar-a-Lago earlier this year, Trump leaned on China to put more pressure on North Korea.

“We've had, I think, positive movement on China over the past five months of this administration, and we'll continue to work with them and others to put the appropriate pressure on North Korea to change this behavior and this regime,” said White House press secretary Sean Spicer at a briefing on Tuesday. “China can play -- has played and can continue to play a greater role in helping to resolve this situation, and we will continue to hopefully build on the relationship and the dialogue that we’ve had with China.”

READ: North Korea Says It Fired New Nuclear Missile, Wants Warhead To Hit US: Report

North Korea’s first nuclear test occurred Oct. 9, 2006, and was a small test of less than one kiloton. One kiloton is equal to around 1000 tons of TNT. The second test was in 2009, and was twice as large as the first. This test prompted the United Nations Security Council to condemn the North Korea, and pass sanctions against the country.

The next nuclear test occurred in 2013, and was more than twice as powerful as the last blast, clocking in at around six to seven kilotons according to the South Korean government. The next two tests were in 2016, the first in January and the second in September. The last test was North Korea’s largest and was around 10 kilotons of force.