A United Airlines flight from San Diego to Newark was forced to return to San Diego International Airport shortly after takeoff because of an external battery device that caught fire on the flight.

The San Diego Fire Department (SDFD) reported on Twitter that an external battery pack caught fire, but the flight crew prevented it from spreading further through the airliner's cabin by placing a fire bag over it. SDFD evaluated all passengers and crew following the incident and said that four patients were transported to the hospital while two others opted not to be transported.

"This morning, United Airlines flight 2664 departed San Diego International Airport heading to Newark. Shortly after departure, the aircraft declared an Alert 2 which indicates a major difficulty with the aircraft, in this case there was an electrical device fire in the cabin," Sabrina LoPiccolo, a spokesperson for San Diego International Airport, told FOX Business. " The aircraft landed safely at San Diego International Airport where emergency crews responded to the aircraft and assisted passengers."

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LoPiccolo noted that no other operations at the airport were impacted by the incident.

United Airlines did not immediately respond to a request for comment.

The Federal Aviation Administration (FAA) requires lithium batteries be stored in carry-on baggage only. 

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The rule applies to spare or uninstalled lithium-ion batteries and lithium metal batteries, including power banks and cell phone battery charging cases. It also covers lithium metal and spare rechargeable lithium-ion batteries for personal electronics such as cameras, cell phones, laptops, tablets, watches, calculators, and more.