Four workers were injured in a fire at the refinery in the northeastern Mexican city of Ciudad Madero, state-owned oil giant Petroleos Mexicanos, or Pemex, said in a statement released Monday.

The fire started while workers were installing a line Sunday at the Madero refinery, which is in Tamaulipas state.

"Regrettably, four workers were injured and they are being treated at the Pemex Regional Hospital in Ciudad Madero," the oil company said.

One of the workers, identified as Faustino Reyna Lara, 46, is listed in serious condition, Pemex said.

The other workers hurt in the accident are Pedro de Leon Martinez, 42, Angel Perez Hernandez, 38, and Sergio Alberto Reyna Lara, 41, the oil company said.

The fire started around 3:30 p.m. Sunday and was "completely extinguished" an hour later, Pemex said.

The blaze did not cause any damage to "the refinery's processing plants, which are operating normally," Pemex said.

An explosion occurred on Aug. 14 at the same refinery, but no one was injured.

Pemex, the world's fourth-largest oil company, produces 2.5 million barrels per day (bpd) of petroleum.

The state-owned corporation is the biggest source of revenue for the Mexican Treasury.

Pemex, Latin America's largest corporation, has a monopoly over the Mexican petroleum industry. EFE