Sancho was named 'Player of the Tournament' at the U-17 Euros earlier this year, where England finished as runners-up after losing the final to Spain on penalties.
Sancho joined the team's first official practice, four days ahead of their Cup opener against Chile.
Sancho did not travel with the team as he arrived separately after getting a last-minute release from Bundesliga giants Dortmund.
"For now, Jadon is here for the group stage and that's all we are focusing on," England media manager Amy Hart told reporters in the sidelines of their practice at SAI training ground.
The London-born teenager has scored 13 goals in 16 games for the Young Lions and is expected to be a key player if they are to progress to the last 16 in a "group of death' featuring two-time champions Mexico, Chile and Asian winners Iraq.