A riot broke out Sunday night outside of a major soccer game in Egypt, killing at least five people as a health official said the death toll was likely to rise.

The violence began ahead of a match between Egyptian Premier League clubs Zamalek and ENPPI at Air Defense Stadium just outside of Cairo.

What caused the violence wasn't immediately clear. Security officials, speaking on condition of anonymity as they weren't authorized to speak to journalists, said Zamalek fans tried to force their way into the match without tickets, sparking clashes.

Zamalek fans, known as "White Knights," posted on their group's official Facebook page that the violence began because authorities only opened one narrow, barbed-wire door to let them in. They said that sparked pushing and shoving that later saw police officers fire tear gas and birdshot.

The group later posted pictures on Facebook it claimed were of dead fans. The Associated Press could not immediately verify the images.

Health Ministry spokesman Hossam Abdel Ghaffar said at least five people were killed and that the death toll was expected to rise. He said some 20 people were wounded.

Egypt's hard-core soccer fans, known as Ultras, frequently clash with police inside and outside of stadiums. They are deeply politicized and many participated in the country's 2011 uprising that forced out President Hosni Mubarak.

During a 2012 match in Port Said, 74 people, mostly fans of the Al-Ahly club, were killed in a riot.