Five Chinese ships have manoeuvred dangerously close to an unarmed US navy surveillance vessel in the South China Sea, the US government says.
US officials said the incident on Sunday came after days of "increasingly aggressive" acts by Chinese ships.
These violated international law on respecting other users of the seas, a Pentagon spokesman said.
As Beijing kept silent, the US made a formal complaint to China's military attache at the Pentagon.
The incident happened as the USNS Impeccable was on routine operations in international waters 75 miles (120km) south of Hainan island, a US statement said.
The ships "aggressively manoeuvred" around the Impeccable "in an apparent co-ordinated effort to harass the US ocean surveillance ship while it was conducting routine operations in international waters", according to the Pentagon.
Impeccable is designed specifically t
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