A burning e-cigarette sparked a fire alert on board a plane, delaying it for several hours.

A passenger's backpack started burning on an Alaska Airlines flight from Seattle in the USA to Anchorage in Alaska and batteries for an e-cigarette are thought to blame.

Flight 67 had just made a scheduled stop in Ketchikan, Alaska, and about half the passengers had got off when the incident happened.

Andrew Hames, who was seated just a few rows behind the passenger, said: "I heard sort of a whooshing sound, like a quick hiss of air, and I looked up and about three rows in front of me a guy's backpack started smoking and burst into orange and blue flame. He quickly got it off and hit it to the ground and some other passengers got up and started stomping on it."

Andrew said the man thought the source of the fire might have been an e-cigarette, but that wasn't the complete answer. After all passengers had stepped off into the Ketchikan airport, Andrew spoke with the man as he emptied out his pack.

"He said it was this device - and held up the charger itself,” he said.

Alaska Airlines spokeswoman Ann Zaninovich said newly charged batteries were to blame.

"Technical experts believe that when the batteries came in contact with metal keys and coins it caused a spark,” she said. “There was visible smoke and a set of keys and candy fell to the ground through a burned hole in the backpack.

"While there was not fire, there was sparking which prompted the flight attendants to take swift action and use the fire extinguisher."

Zaninovich said that "out of an abundance of caution," the aircraft's first officer put "the device" in a fire containment bag which is carried on all of the airline's planes.

* A pioneering invention called an AvSax can minimise the fire danger in seconds with its unique use of water.

Simply pour at least two litres of water into an AvSax and then drop the burning device into the bag. The water activates a polymer gel inside the bag’s lining causing it to expand around the device. Should the device keep on burning then the AvSax is tough enough to absorb the force.