A China Coast Guard vessel sounded an alarm as Philippine boats conducted a research mission in contested waters in the South China Sea.

Footage shows the Chinese ship blaring its siren to warn crew and researchers aboard the Philippines' BRP Datu Sanday on March 21.

The Philippine ship was part of a contingent conducting marine research on two barren sandbars called Sandy Cay.

China Coast Guard (CCG) ships and a military helicotper reportedly tried to block the Philippine government vessels.

CCG spokesman Gan Yu said in a statement that the Chinese personnel dealt with 'illegal activities' by Philippine staff who 'ignored China's warnings and dissuasion.'

Meanwhile, Philippine Coast Guard spokesperson Commodore Jay Tarriela said the research mission was a success despite the 'dangerous manoeuvres' of the CCG.

He said: 'Amid their (Chinese) dangerous maneuvers, blocking operations, and the Chinese maritime militia's illegal presence there, we were able to complete our marine scientific research.'

China currently lays claim over almost the entire South China Sea, one of the world's busiest sea lanes. But a United Nations-backed tribunal in 2016 deemed its claims groundless.