The US aircraft, a WC-135 Constant Phoenix, was conducting a "routine mission" in international airspace on May 17 when it was intercepted by two Chinese Sukhoi SU-30 fighter aircraft, the Air Force said in a statement.
The WC-135 is a so-called "sniffer plane" designed to scan the atmosphere for signs of nuclear activity.
"The WC-135 was operating in accordance with international law. While we are still investigating the incident, initial reports from the US aircrew characterised the intercept as unprofessional," the Air Force said.
The US was addressing the issue with China through "appropriate diplomatic and military channels".
Chinese foreign ministry spokesperson Hua Chunying told a regular press briefing in Beijing that she had no information about the incident, and the country's ministry of defence did not respond to AFP's request for comment.
Mid-air interceptions occur routinely in international airspace, but the US military will often call out foreign pilots if it judges the manoeuvers to be risky or unprofessional.