The United States government is complaining to China about an incident involving a naval vessel in international waters.
The US Defence Department says five Chinese ships harassed an unarmed US navy ocean surveillance ship in international waters while on routine operations in the South China Sea, 120km south of Hainan Island, on Sunday.
A Defense Department statement said one vessel came within 7.6 meters of the USNS Impeccable.
The Impeccable is one of six surveillance ships that gather underwater acoustical data.
The Pentagon says there have been half a dozen such incidents since 4 March, including fly-bys by maritime surveillance aircraft and a close encounter with a frigate, at a range of 30.4 meters on 5 March.