Ree-Zammit, who has won 42 caps on the wing for Wales, has been training with the two-time Super Bowl winners for the last couple of months since taking part in a 10-week training camp as part of the NFL's International Player Pathway (IPP).
During the pre-season games in August, Rees-Zammit was used sparingly in several positions, with modest statistics (22 yards in six carries, three yards on one reception), according to the American press.
Rees-Zammit is expected to continue training with the Chiefs although he is also allowed to sign for another club.
Rees-Zammit stunned the rugby world in January, on the eve of the Six Nations, when he announced that he was giving up the game to pursue his dream of playing in the NFL.
Winners of three of the last five Superbowls, the star-studded Chiefs - Patrick Mahomes is at quarterback while Taylor Swift's boyfriend Travis Kelce plays tight-end - kick off their season at their Arrowhead Stadium on September 6th against the Baltimore Ravens.
