(Reuters) – Icing problems that have occurred on Boeing Co 787 Dreamliner and 747-8 aircraft using engines made by General Electric Co could lead to forced landings, the U.S. aviation regulator said on Wednesday.

The so-called airworthiness directive from the Federal Aviation Administration had been expected after a series of incidents this year. The FAA said the problems affected nine flights and in two cases involving 747-8 planes it affected two of four engines.

The directive provided more details about icing that the agency said could occur in some types of high-altitude thunderstorms. The icing could reduce the ability of the engines to provide thrust and “could lead to a forced landing,” the agency said. The incidents all occurred in 2013.

The problem became known last Friday, when Boeing said it had issued a notice to airlines about the risk of engine icing and the loss of thrust that could result. Boeing and GE had said there were six incidents in which icing occurred, but the FAA’s notice cited nine.

The FAA tally was higher because it included incidents in which the engines “experienced a vibration during the ice-crystal storms, but didn’t lose any thrust,” GE Aviation spokesman Rick Kennedy said in an email.

Japan Airlines Co Ltd has said it is replacing 787 Dreamliners on two routes because of the problem. United Airlines, the operating unit of United Continental Holdings Inc and the only U.S. carrier currently flying the 787, said it was not making changes because of the issue.

Boeing and GE both said they are working closely with the FAA to resolve the issue.

SOURCE REUTERS