Search and rescue operations were underway for five crew members missing after a US Navy helicopter crashed into the sea off the coast of California, the US Pacific Fleet said on Tuesday.

One crew member had been rescued but five were missing, the navy fleet said in a statement.

The MH-60S helicopter took off from the USS Abraham Lincoln during "routine flight operations" about 60 nautical miles off the coast of San Diego at 4:30 pm (2330 GMT), the fleet said.

"Search and rescue operations are ongoing with multiple Coast Guard and Navy air and surface assets," it added.