MEXICO CITY - The death toll from a powerful blast at a petrochemical plant in Mexico has risen to 13, and the final toll is feared to be higher still, an official said Thursday.

"We have searched the affected area within the complex, and sadly so far located 13 victims," Luis Felipe Puente, a senior civil protection official within the interior ministry, said on Twitter.

Three people were previously confirmed to have died in Wednesday's explosion in southeastern Veracruz state that also injured 136.

The explosion, so strong it was felt 10 kilometers (six miles) away, marked the latest in a string of deadly accidents to plague the state-run oil giant Pemex in recent years. This same plant suffered a blast in 1991 that officially left six dead, although local media said the toll was much higher.

In 2012, a Pemex natural gas plant explosion killed 30 people in the northeastern state of Tamaulipas. Even its headquarters - a skyscraper in the heart of Mexico City - was hit in January 2013 by a blast caused by a gas buildup that left 37 dead.

Accidents have taken place at several oil rigs in the Gulf of Mexico while fires have erupted in pipelines after fuel thieves punctured them to steal the product.

The Mexican government has implemented a sweeping reform of the energy sector, which opens it to foreign investors for the first time in decades and partly aims to help modernize the country's aging infrastructure.