Two Chinese Sukhoi Su-30 jets have conducted an “unprofessional” intercept of a US aircraft, the US military said.
One of the Chinese jets came as close as 150ft (45m) to the US WC-135 plane and flew upside down above it, according to US officials cited by CNN.
The US says the plane was on a mission to detect radiation in international airspace over the East China Sea.
Tensions have repeatedly risen over US activity near the resource-rich international waters off China’s coast.
The intercept, which took place on Wednesday, was deemed unprofessional “due to the manoeuvres by the Chinese pilot, as well as the speeds and proximity of both aircraft,” Air Force spokesman Lt Col Lori Hodge said.
She said the issue was “being addressed with China through appropriate diplomatic and military channels”, and a military investigation was under way.
China has not commented on the incident, but it accuses the US of carrying out reconnaissance flights over Chinese coastal waters and regularly calls on the US to reduce patrols in the area.
China claims sovereignty over almost all of the disputed territory in the South and East China seas, though several other countries in the region have competing claims.
China has backed its expansive claims with island-building and naval patrols.
The US sniffer aircraft has previously been used to detect evidence of possible nuclear tests by North Korea.