New rules have gone into effect in the US that prohibit air passengers there from carrying spare lithium ion batteries in their checked baggage.

The new rules are designed to reduce the risk of fires in aircraft - such batteries have been identified as a possible cause of several aircraft fires.

Passengers will still be able to carry batteries in checked bags if they are installed in a device like a laptop or digital camera. But loose batteries must be put in a plastic bag and carried on the plane as hand luggage.

The rules also limit each passenger to two "extended-life" lithium batteries. These are larger batteries with more than 8 grams of equivalent lithium content, examples of which are pictured in the DOT's statement.

In February 2006 a United Parcel Service flight landed at Philidelphia International Airport after the crew detected a fire in its cargo. The National Transportation Safety Board said later that it found several burned out laptop batteries on the plane, and could not rule them out as a possible cause of the fire.

Lithium batteries are a fire hazzard because of the heat they can generate when they are damaged or suffer a short circuit, the NTSB said at a hearing about the Philidelphia incident last July.

Several lithium battery incidents have occurred in recent years, including a lithium-ion battery fire that occurred less than two months ago on an airplane in Chicago.