A rotary assembly, for FFS (Form, Fill and Seal) machines, for ultrasound sealing a continuous tubular strip housing an orderly succession of products and travelling along a feed path extending through the sealing station; the rotary assembly having two rotors, which are located on opposite sides of the feed path, and rotate in opposite directions about respective axes; and at least one of the two rotors being fitted with at least one ultrasound sealing device, which cooperates at the sealing station with a corresponding anvil fitted to the other rotor with the interposition of elastic members.