Two Chinese Sukhoi Su-30 jets have conducted an “unprofessional” intercept of a US aircraft, the US military said.
The US WC-135 plane was on a mission to detect radiation in international airspace over the East China Sea.
One of the Chinese jets came as close as 150ft (45m) to the US aircraft, according to US officials cited by CNN.
China’s suspicion of US activity near the resource-rich international waters off its coast has repeatedly led to tensions between the two countries.
“The issue is being addressed with China through appropriate diplomatic and military channels,” Air Force spokesperson Lt Col Lori Hodge said.
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 intercept was deemed unprofessional “due to the manoeuvres by the Chinese pilot, as well as the speeds and proximity of both aircraft,” Lt Col Hodge added, saying a military investigation was underway.
The US sniffer aircraft 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.
It 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 verbally clashed over activity in the South China Sea.
In February, a US aircraft carrier started what Washington described as “routine operations” in the South China Sea, with a fleet of supporting warships.
The deployment came despite Chinese warnings against challenging Beijing’s sovereignty in the region.
In May 2016, two Chinese fighters carried out a similar intercept of a US military aircraft over the South China Sea.
At the time, the US military said its maritime reconnaissance aircraft was carrying out a routine patrol in the area.