The National Transportation Safety Board is issuing a rare urgent safety bulletin warning of potential problems with the jet engines on Boeing 737 Max aircraft.

US safety investigators called for urgent action to be taken in response to an engine issue on Boeing Co. 737 Max aircraft that can lead to smoke in the cockpit or cabin.The National Transportation Safety Board’s recommendation comes after at least two incidents in which planes were filled with smoke after large birds struck their engines.

The NTSB issued an “urgent safety recommendation” related to the engine on the 737 MAX after two incidents where smoke entered the cabin.