Mexico airport operators Grupo Aeroportuario del Pacifico SAB and Grupo Aeroportuario del Sureste SAB cautioned investors that the government has changed their concession agreements.

The national aviation agency “unilaterally and without prior communication” modified the fee structure related to its airports, GAP said in a statement sent after hours to the local regulator. Both GAP and Asur said in filings that they are evaluating the effect of the changes on their operations. A large part of airport operators’ revenues in Mexico come from so-called TUAs, or passenger fees for airport use.

It wasn’t immediately clear what changes the government had made, and the companies didn’t elaborate. Authorities have in recent weeks increased their activity in the sector, with the government announcing the revival of state-run airline Mexicana de Aviacion, and transferring the operation of Benito Juarez International Airport to the Navy following an announcement in June.

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A spokesman for the infrastructure, communications and transport ministry didn’t immediately reply to a request for comment.

--With assistance from Jose Orozco.