US spy planes watching for possible North Korea nuke test

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The U.S. continues to take active measures to prepare for the possibility of a nuclear test by North Korea, U.S. officials told NBC News, including the deployment of a special nuclear "sniffer" aircraft, drones, and U-2 spy planes.

The U.S. Air Force's special sniffer plane is capable of detecting evidence of a nuclear explosion.

In a statement, the Air Force said the aircraft, known as a WC 135, "has been deployed on a routine mission to Northeast Asia. These missions are planned well in advance."

RELATED: Satellite images of the Punggye-ri nuclear test site in North Korea

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Satellite images of the Punggye-ri nuclear test site in North Korea
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Satellite images of the Punggye-ri nuclear test site in North Korea
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - APRIL 2, 2017. Figure 1. Activity continues at the North Portal. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - APRIL 2, 2017. Figure 2. Possible new dumping observed at the North Portal spoil pile. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - APRIL 2, 2017. Figure 3. Probable personnel in formation or equipment in rows at the Main Administrative Area. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 30, 2017. Figure 1. No vehicles or trailers remain around the North Portal but well-worn paths are observed. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 30, 2017. Figure 2. No new dumping of material on the North Portal spoil pile. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 30, 2017. Figure 3. Small collection of crates or trailers seen in previous imagery has been removed. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 28, 2017. Figure 3B. Formations seen in the Main Administrative Area, similar to what was seen in lead up to 2013 nuclear test. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 28, 2017. Figure 2. Material dumped at the North Portal tailings pile. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - JANUARY 4, 2013. Figure 3A. Formations seen in the Main Administrative Area in lead up to 2013 nuclear test. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 28, 2017. Figure 1. Continued activity at the North Portal. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 25, 2017. Figure 1. Probable cabling and water drainage seen at the North Portal. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - OCTOBER 19th, 2016: Figure 6: Excavation continued underground in the North Portal area suggesting more tests to come in the same tunnel complex directly under Mt. Mantap. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - JANUARY 5th, 2017: Figure 7: The North Portal spoil pile continued to expand into 2017, becoming increasingly broader. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - JANUARY 22nd, 2017: Figure 8: Late January 2017 imagery showing new spoil on top of recent snow. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - OCTOBER 19th, 2016: Figure 9. A close-up of the North Portal spoil pile as it appeared in late October 2016. The unstable spoil can sometimes lead to accidents, as in this case of toppled rail cars downslope. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - FEBRUARY 12th, 2017: Figure 10. A close-up of the North Portal spoil pile from February 2017 shows that accumulations had begun move westward with a broadening of the top and bottom west side of the pile. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 7th, 2017: Figure 1. DigitalGlobe imagery showing large shipping container or crate seen at the North Portal. Date: March 7, 2017. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 7th, 2017: Figure 2. DigitalGlobe imagery showing no changes to pattern and texture of tailings (spoil) pile at the North Portal. Date: March 7, 2017. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 7th, 2017: Figure 3. DigitalGlobe imagery showing a small vehicle present at the West Portal. Date: March 7, 2017. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 7th, 2017: Figure 4. DigitalGlobe imagery showing a truck present in the southern courtyard of the Main Administrative Area. Date: March 7, 2017. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 7th, 2017: Figure 5. DigitalGlobe imagery showing a truck present at the sites Command Center. Date: March 7, 2017. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - MARCH 7th, 2017: Figure 6. DigitalGlobe imagery showing snow cleared at guard barrack and security checkpoint. Date: March 7, 2017. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - OCTOBER 24, 2016: Figure 2. No activity seen at the Sohae launch pad. Date: October 24, 2016. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - OCTOBER 24, 2016: Figure 3. Environmental shed remains adjacent to the vertical engine test stand. (Photo DigitalGlobe/38 North via Getty Images)
PUNGGYE-RI NUCLEAR TEST SITE, NORTH KOREA - OCTOBER 29, 2016: Figure 1C. Increased activity around the North Portal throughout October. Date: October 29, 2016. (Photo DigitalGlobe/38 North via Getty Images)
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The Air Force said that the WC 135 flies about 150 missions annually at various locations around the world, and "is designed to collect accurate information on levels of radiation in the atmosphere in order to monitor compliance with international treaties governing nuclear testing."

Related: U.S. May Not Be Able to Shoot Down North Korean Missiles

Two U.S. intelligence officials told NBC News that North Korea is in a position to conduct a nuclear test with little or no warning.

"We expect that they are able to do it at any time, without any real warning," one official said. "They have been engaging in steady preparations."

Related: Trump's Options for North Korea Include Placing Nukes in South Korea

The officials would neither confirm nor deny that the U.S. has detected signs that a test is imminent.

Three U.S. military officials said the Chinese military is currently on a higher alert as part of a nationwide exercise. The exercise is defensive in nature and not focused on the region near North Korea.

During a Thursday press conference with Italian Prime Minister Paolo Gentiloni, President Trump called North Korea a "menace." He answered a reporter's question about instability in North Korea by praising China's president and China's actions in the region.

"As far as North Korea is concerned, we are in very good shape," said Trump. "I respect [President Xi Jinping] very much, and I think he's working very hard... Some very unusual moves have been made over the past two or three hours and I really have confidence that the president will try very hard."

RELATED: Major cities in range of North Korea's missiles

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