The American plane was flying within in global airspace and thus in accordance with worldwide law, two USA military officials told the news site. The aircraft is able to monitor nuclear weapons testing and can track radioactive activity by collecting samples of particles and chemical substances.
Whether the close encounter was meant to send a signal to the Americans or was merely the action of an over exuberant pilot is not clear, experts said.
Hodge also said that most military aircraft interactions, including most U.S.
Dubbed the "Constant Phoenix", the four-engine WC-135 jet looks for distinctive elements a nuclear test of any type would emit into the air.
Chinese foreign ministry spokeswoman Hua Chunying declined to comment on the specific incident and referred questions to the defence ministry which has yet to comment. It is deployed to Northeast Asia in advance of possible North Korean underground nuclear tests. The U.S. opposes China's territorial claims in the region, and routinely asserts its right to fly or sail in areas it considers global seas or airspace.
"The issue is being addressed with China through appropriate diplomatic and military channels", air force spokeswoman Lieutenant Colonel Lori Hodge said on Thursday.More news: Donald Trump signals shift in US' Mideast line
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NBC said the crew of the U.S. plane described the encounter as "unprofessional", although not necessarily risky.
Meanwhile, China urged the USA to halt such reconnaissance exercises in order to prevent future incidents.
The ministry, in a short statement, said the U.S. account of the incident did "not accord with the facts", and urged the United States to cease close-in surveillance flights.
The Japanese coast guard observed what appeared to be a drone flying close to a Chinese vessel that was sailing in Japanese territorial waters, Defense Minister Tomomi Inada said on Friday. A radiation detection aircraft was dispatched to Japan in early April, and several sorties have since been flown in the region.
In 2013, China mapped an air defense identification zone over a vast area of the East China Sea, USA Today cited. "This incident is an extremely serious breach of our country's sovereignty".
China has backed its expansive claims with island-building and naval patrols.