A search and rescue operation was launched after the MH-60S helicopter crashed around 4:30 p.m. about 60 nautical miles off San Diego, the Navy's U.S. Pacific Fleet said in an initial statement.
Officials later said that one crewmember had been rescued and "search efforts continue for five additional crewmembers."
The crash happened while the aircraft was "conducting routine flight operations," according to the Navy. It had been aboard the USS Abraham Lincoln.
"Search and rescue operations are ongoing with multiple Coast Guard and Navy air and surface assets," the tweet said.
No other details were immediately available.
The MH-60S is a versatile aircraft that typically carries a crew of about four and is used in missions including combat support, humanitarian disaster relief and search and rescue.