Investigators are looking for more information about an incident in which a United Airlines plane dropped to within 250 metres of the ocean surface after taking off from Hawaii.

United says the pilots are getting additional training.

The Boeing 777 dropped more than 470 metres before climbing again, according to data from tracking service Flightradar24.

The plane then continued on to San Francisco.

No injuries were reported.

A spokesman for the National Transportation Safety Board in the United States said on Monday, local time, that the investigative agency was still seeking information about the December 18 incident, which only recently came to public attention after a report in The Air Current, an aviation-industry publication.

The Federal Aviation Administration (FAA), which oversees airlines, said the United crew reported the incident under a voluntary safety-reporting program.

The FAA said it reviewed the incident "and took appropriate action" without providing further details.

United said it worked with the FAA and the pilots' union on an investigation that led to additional training for the two pilots, which is still going on.

The airline did not explain why the pilots chose to continue the long, overwater flight to San Francisco instead of returning to Kahului Airport on the island of Maui.

AP