WASHINGTON -- The Federal Aviation Administration is warning U.S. carriers about the risk of fires caused by e-cigarettes.

Earlier this month at Los Angeles International Airport, an overheated e-cigarette sparked a fire in a piece of luggage in a baggage area. Four months prior to that at Boston's Logan Airport, an e-cigarette in a passenger's bag caught fire in the cargo hold of a plane -- forcing an evacuation.

Those incidents and others have prompted the FAA to issue an official warning to U.S. carriers. The agency says the popular tobacco alternative "can pose a fire hazard in the cargo compartment of planes."

Fires can spread when lithium ion batteries ignite. The FAA has already cracked down on those but e-cigarettes are powered by the same technology: Lithium cells.

The concern is that e-cigarettes could inadvertently turn on in checked luggage, igniting a fire that could be catastrophic in mid-flight.

This frame grab from video, provided by the Federal Aviation Administration (FAA) shows a test at the FAAs technical center in Atlantic City, N.J. last April, where a cargo container was packed with 5,000 rechargeable lithium-ion batteries. AP / FAA

The FAA is not banning e-cigs on planes outright. It just wants passengers to carry them onto the plane so that they can be monitored.