United States defender Ventura Alvarado joined Club America's youth team in Mexico in a possible breach of FIFA's Article 19. iguel Tovar/LatinContent/Getty Images

The Mexican Football Federation has encouraged Liga MX clubs to stop registering underage players from the United States because of a potential widespread breach of a FIFA rule, sources have told ESPN FC.

Each year, minors born and based in the U.S. are scouted north of the border and head south to join Liga MX clubs' academies.

But sources said the Mexican teams could be guilty of breaking Article 19 of FIFA's Regulations on the Status and Transfer of Players, which is designed to stop minors from leaving their native countries for bigger clubs abroad before they turn 18.

There are between 20-30 such players currently in Mexico, according to one source.

Article 19 is the same rule for which Barcelona received a transfer ban and Real Madrid and Atletico Madrid received suspended bans.

And the Mexican federation's instruction to teams came about precisely because of the attention the cases of the Spanish clubs have shone onto the issue of Article 19, sources said.

The regulation says: "International transfers of players are only permitted if the player is over the age of 18," though there are three exceptions to the rule:

- The player's parents move to the country in which the new club is located for reasons not linked to football.

- The transfer takes place within the territory of the European Union (EU) or European Economic Area (EEA) and the player is aged between 16 and 18.

- The player lives no further than 50 kilometers from a national border and the club with which the player wishes to be registered in the neighboring association is also within 50 kilometers of that border.

But none of those exceptions appear to validate United States national team defender Ventura Alvarado moving from Phoenix to join Club America's youth academy as a 15-year-old, for example.

Neither do they apply to U.S. youth international Joe Gallardo moving from California to Monterrey and playing in official matches as a 15-year-old. Or Juan Pablo Ocegueda's signing with Tigres from California as a minor.

A check of the Liga MX website reveals that at least five U.S.-based players have featured in official Liga MX youth teams in 2016. Add up all the players under the age of 18 who have signed for Liga MX teams going back to 2003 and the potential breach would be widespread.

A FIFA spokesperson told ESPN FC that the world's governing body is "aware" of the issue of American minors playing in Mexico, but added that "we are not in a position to comment on any investigations that are ongoing so as not to compromise the process, nor do we provide any comments as to whether or not any investigations are underway."

FIFA also wouldn't comment on how these international transfers were registered considering that since Oct. 1, 2010, every transfer across federation lines has had to be done via the online Transfer Matching System (TMS). One of the principal reasons for the creation of the system was the protection of minors and to make sure players under the age of 18 could not be transferred abroad.

In February, former Chivas youth director Jose Luis Real -- who has since left the club -- revealed that Chivas had not been able to register five players from the United States between the ages of 13 and 16 for their youth teams this season.

The Mexican federation directed ESPN FC to Liga MX's press secretary for comment, but there was no response.