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 Lieutenant Colonel Lori Hodge said.
The intercept was deemed unprofessional "due to the manoeuvres by the Chinese pilot, as well as the speeds and proximity of both aircraft," she 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.
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.
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