RIO DE JANEIRO -- The first phase of ticket sales for the Rio Olympics in 2016 began on Tuesday despite uncertainty about the football tournament.

Nearly 7.5 million tickets are available to fans, but none for football games in Sao Paulo because the city and local Olympic organizers were not able to reach a deal ahead of the opening of the sales.

Sao Paulo officials are still analyzing the costs of hosting the tournament and can't guarantee the matches will go ahead, while Olympic organizers are preparing a Plan B in case the city's games in the tournament have to be staged elsewhere.

Fans will have about a month to choose which events they want to watch and a random draw in June will determine who gets the tickets. A second draw will follow later in the year.

About 70 per cent of the tickets are reserved for Brazilians.

International fans will buy tickets directly through an official reseller in their own country. The resellers were announced by Rio 2016.

Ticket prices start at about $12 ((euro)11), with more than half costing less than $25 ((euro)23). The most expensive ticket, for the opening ceremony on Aug. 5, costs almost $1,400 ((euro)1,300).

Brazilians are entitled to discounts for students and senior citizens. They are also allowed to pay for tickets in up to five monthly installments. International resellers can charge up to 20 per cent in service fees.

Although local Olympic organizers and FIFA confirmed Sao Paulo as one of the six hosts for the football tournament, along with Rio, Brasilia, Salvador, Belo Horizonte and Manaus, the city still can't guarantee to stage any matches because of the costs involved.

Tickets for the games in Sao Paulo -- or the city replacing it in the tournament -- may not become available until the final online sales stage. Football was one of the most sought-after sports by fans who registered for tickets, according to local organizers.

On Tuesday, Olympic organizers for the first time admitted they are considering a Plan B in case Sao Paulo is dropped.

"We have several versions of the football match schedule," Rio communications director Mario Andrada said, adding that work continues to "make sure a deal with Sao Paulo can be reached as soon as possible."