The state-run oil company initially reported that three workers were injured but later raised the number.
Veracruz state Gov. Javier Duarte's Twitter account called the blast at the plant in the city of Coatzacoalcos a "strong explosion."
Mexico's Gulf port of Coatzacoalcos in the state of Veracruz was closed on Friday due to 82 kilometer-per-hour winds spurred by a cold front,
The zones include the Port of Lázaro Cardenas, the neighboring municipalities of Michoacán and Guerrero as well as the Interoceanic Corridor of Tehuantepec, including Coatzacoalcos, Salina Cruz and Puerto Chiapas.
El aseguramiento se realizó empleando equipo de rayos “X” en el punto de inspección instalado en el tramo Coatzacoalcos de la carretera Federal número 180