U.S.-based Southwest Airlines is investigating reports of unusual vandalism on the underbellies of its planes.

Southwest Airlines spokeswoman Whitney Eichinger told VOA Thursday that markings are being found during routine inspections of the airline's Boeing 737 aircraft.

U.S. media reports say the markings appear to be Arabic or Arabic-type symbols. They say the symbols were apparently made using some kind of chemical process, and only become visible after an auxiliary power unit is turned on and heats up the outside of the aircraft.

The Southwest spokeswoman said the airline began to notice the markings in February. She did not specify how many planes have them, but said the vandalized aircraft are still in use.

Southwest says it is taking the matter very seriously, but does not believe it is a flight safety issue.

Both the Transportation Security Administration and FBI are working with Southwest on the matter.