BP on Monday restarted operations to stream dispersants directly into the main Gulf of Mexico oil leak despite fears the chemicals could themselves be harmful to the environment.

A 1.6-kilometre-long tube was fed down to the leaking pipe on the sea floor and shot the dispersant directly into the flow, guided by remotely-operated robotic submarines.

State and federal agencies "consented to the third test today of subsea dispersant", BP spokesman John Curry said.

The test began at 4.30am "and will continue for 24 hours. After the test is concluded, further evaluations will be conducted", Curry said.