"The issue is being addressed with China through appropriate diplomatic and military channels," said Air Force spokeswoman Lieutenant Colonel Lori Hodge.
Hodge said the U.S. characterization of the incident was based on initial reports from the U.S. aircrew aboard the WC-135 Constant Phoenix aircraft "due to the maneuvers by the Chinese pilot, as well as the speeds and proximity of both aircraft."
"Distances always have a bearing on how we characterize interactions," Hodge said, adding a U.S. military investigation into the intercept was underway.
She said the WC-135 was carrying out a routine mission at the time and was operating in accordance with international law.
Chinese foreign ministry spokeswoman Hua Chunying declined to comment on the specific incident and referred questions to the defense ministry which has yet to comment.
"For a long time U.S. ships and aircraft have been carrying out close up surveillance of China which can really easily cause misunderstandings or misjudgments or cause unexpected incidents at sea or in the air," she said. "We hope that the U.S. side can respect China's reasonable security concerns.
"But as for this question you have raised and are concerned about, we need to understand and check what was the actual situation on the scene," she said, without elaborating
On Feb. 8, a U.S. Navy P-3 spy plane and a Chinese military aircraft came close to each other over the South China Sea in an incident the Navy saw as unsafe but also inadvertent.
Reuters reported at the time that the aircraft came within 1,000 feet (305 meters) of each other in the vicinity of the Scarborough Shoal, between the Philippines and the Chinese mainland.
China is deeply suspicious of any U.S. military activity in the resource-rich South China Sea, parts of which are disputed by China and its smaller neighbors including the Philippines, Vietnam and Malaysia.