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US air safety officials say they will investigate why a Boeing 777 jet unexpectedly lost altitude and nearly plunged into the Pacific Ocean.

United Airlines flight 1722 had left Maui on 18 December and was climbing when it suddenly plummeted 1,400ft (425m), reports say.

It stabilised at just 775ft in altitude and went on to land safely in San Francisco 27 minutes early.

Its adds to a number of near misses involving planes this year.

The flight was going normally until 71 seconds after take-off from Kahului Airport when it suddenly dropped in elevation, according to Flightradar24, an aviation website.

The National Transportation Safety Board (NTSB), which investigates US plane crashes, will produce a report about the incident within three weeks.

Passenger Rod Williams told CNN: "There were a number of screams on the plane. Everybody knew that something was out of the ordinary, or at least that this was not normal."

He said it was "sobering" to think they were probably about five seconds away from hitting the water.

The flight's pilots filed an internal safety report after landing, a United Airlines representative told ´óÏó´«Ã½ News. They have a combined 25,000 hours of flying experience.

An investigation by the airline resulted in additional training for the pilots, which United said was ongoing.

"Safety remains our highest priority," a company official said in a statement.

The Federal Aviation Administration (FAA) also received a report of the incident, the federal agency says, and "took appropriate action". It did not elaborate.

The incident is among a number of potentially dangerous events reported by US airlines recently, including two near misses in New York and Texas this year.

The NTSB is already looking into two close calls within the last month.

On 4 February, a FedEx cargo plane aborted its landing to avoid a Southwest Airlines plane at Austin-Bergstrom International Airport in Texas.

At John F Kennedy Airport in New York in January, a Delta flight stopped short on the runway during take-off to avoid an American Airlines plane.