Two Chinese Sukhoi Su-30 jets have conducted an “unprofessional” intercept of a US aircraft, the US military said.
The US sniffer plane was on a mission to detect radiation while flying in international airspace over the East China Sea.
The aircraft has previously been used to detect evidence of possible nuclear tests by North Korea.
“The issue is being addressed with China through appropriate diplomatic and military channels,” officials said.
The intercept was deemed unprofessional “due to the manoeuvres by the Chinese pilot, as well as the speeds and proximity of both aircraft,” Air Force spokesperson Lieutenant Colonel Lori Hodge said according to Reuter news agency.
She added that a military investigation was underway.
China is extremely suspicious of any US military activity near the resource-rich international waters off its coast.
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.