American Airlines flight 168, encountered unanticipated moderate to severe turbulence during cruise at 34,000 feet (FL340) while enroute to John F. Kennedy International Airport (JFK), New York, New York from Tokyo Haneda International Airport (HND), Tokyo, Japan. As a result, two flight attendants (FAs) sustained serious injuries. Before departing, the flight crew stated they all reviewed the weather forecast on their weather briefing app before departure and noted no significant turbulence or convective activity. The flight departed HND in Visual Meteorological Conditions (VMC) and climbed on the assigned standard instrument departure procedure and leveled off at FL340 in VMC above an overcast layer. The flight crew stated the flight was smooth during the climb out, with occasional “very light” turbulence. During cruise, the captain and the first officer recalled monitoring the weather radar and did not detect any convective activity in the area. Approximately 30-40 minutes after departure, the overcast area rose into the airplane’s altitude. At that time, the FAs were moving around the galley and cabin for service. The captain made an announcement for the FAs to be seated, but the airplane encountered moderate to severe turbulence. After the turbulence subsided, the captain was informed that two FAs had been injured and were unable to continue their duties. Neither requested immediate medical attention. The flight continued uneventfully to JFK, where the injured FAs were met by company personnel and later sought medical evaluation. One FA was diagnosed with a fractured tibia, and another with a fractured ankle. Two additional FAs later reported minor injuries. No passengers were injured.