US spy planes watching for possible North Korea nuke test

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."

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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."

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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."

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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."

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