FIFA admitted on Tuesday that it was "very difficult" to ensure that teams did not field overage players at the World Youth Championship.
FIFA secretary general Michel Zen Ruffinen said official passports were used as proof of players' ages but admitted that the false passports scandal in Europe had left the system open to questioning.
"It's a very difficult matter," he said.
"We asked each team to provide the players' passports, which are official government documents, and we have also asked for the birth certificates so we have two official documents for each player. If not, there is a tough sanction."
"I don't know what else we can do."
Some officials and media at the World Youth Championship, which is limited to players who were aged 20 or under in January this year, have complained that some players look considerably older.
The competition continues on Wednesday with Argentina meeting Paraguay and Ghana taking on Egypt in the semifinals.
