SAN FRANCISCO (CBS SF) – A United Airlines 747-400 bound for Japan was forced to return to San Francisco International Airport Monday after reports of flames coming out of one of its engines while taking off.

Flight 837 left San Francisco International Airport at 11:36 a.m. headed to Tokyo’s Narita Airport with 202 passengers and 15 crewmembers aboard, United spokeswoman Erin Benson said.

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The 747 began experiencing problems in one of its four engines and had to turn around and come back, Benson said.

Officially, United said there was a mechanical issue with one of the engines, more technically a spokesman called it a “compressor stall”.

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Bay Area resident Sam Smith tweeted that he saw flames coming out of one of its four engines as it was taking off.

“The @united flight out of @flySFO had flames coming out of left side engine, made loud bang noise, and then looked like it was going to land,” he tweeted.

However, the airline could not confirm reports of fire or smoke coming from the plane at any point of the flight.

The jet circled off the Northern California coast for more than an hour burning off fuel and preparing to return to the airport. With emergency vehicles stationed nearby, the flight successfully landed at SFO at 1:35 p.m.

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United is working to place the passengers on other flights, Benson said.