Southwest Airlines is investigating mysterious markings, not seen as a threat, that began appearing on the bellies of its jets earlier this year and have been described as "similar to Arabic writing," an airline spokeswoman said on Thursday.

"The markings have no affiliation to any known group or activity," Southwest spokeswoman Brandy King said. "The markings at this time are being considered vandalism, and Southwest is conducting an internal investigation to determine who is responsible."

She said the markings have been appearing on the airline's signature 737 jets for the past several months and vary in appearance.

The FBI and other law enforcement agencies all concluded that the markings, described "as similar to Arabic writing," do not present a threat or a safety hazard, King said.

Officials have not confirmed whether the markings are messages of any sort, or if they are words or simply scribbling. King said it was also not known if they were made on purpose or were the result of some type of mechanical issue.

She declined to elaborate further on the markings.

King said Southwest Airlines, which exclusively uses 737 aircraft in its routes, had determined that there was no danger to the planes or the passengers.

"We obviously take the behavior very seriously and we will continue to involve local and federal law enforcement as needed until the situation is resolved," King said.