The death toll from the explosion last week at the Petroleos Mexicanos, or Pemex, headquarters in Mexico City has risen to 36, President Enrique Peña Nieto said.

Rescue workers pulled three bodies out of the rubble over the weekend and are still searching for one person, the president said in a Twitter posting.

"My deepest condolences go out to the families of all the victims," Peña Nieto said.

Pemex's headquarters was torn by a blast last Thursday that occurred in the basement of one of the complex's buildings.

"Regrettably, the number of people killed has risen to 36," Pemex said in a statement.

Some 2,500 rescue workers - aided by specially trained dogs - sifted through the rubble at Pemex headquarters over the weekend.

The cause of the blast has not yet been determined and, according to official reports, investigators have not found indications of a fire, smoke nor an odor of flammable substances.

The main tower of the office complex, a 56-story structure, was not damaged in the blast. EFE