A US aircraft has been “unprofessional” intercepted by two Chinese Sukhoi Su-30 jets, the US military said.
One of the Chinese jets came as close as 150ft to the US WC-135 plane and flew upside down above it, according to US officials cited by CNN.
According to the US military, 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 May 17, was deemed unprofessional “due to the maneuvers by the Chinese pilot, as well as the speeds and proximity of both aircraft”, Air Force spokesman Lt. Col. Lori Hodge said.
He 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.
It 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 plane has previously been used to detect evidence of possible nuclear tests by North Korea.
Separately, China and South East Asian countries have agreed a framework for a long-awaited code of conduct for the disputed South China Sea, China’s foreign ministry said, without giving details on the content.
The draft will now be submitted to the foreign ministers of the countries in August.
Over the years, the US, China and several of the South East Asian countries have had disputes over rival activity in the South China Sea.