Coatzacoalcos (Mexico) (AFP) - The death toll from a thunderous blast at a petrochemical plant in Mexico has risen to at least 13, officials said Thursday, in the latest accident plaguing state-run oil giant Pemex.

A cloud of toxic chemicals spewed from the plant after Wednesday's explosion, which sent huge plumes of thick, black smoke billowing into the sky.

Pemex insisted the toxic cloud posed no threat to the population, but the explosion triggered panic among locals still traumatized by a 1991 blast at the same plant that caused a deadly gas leak.

The death toll could rise further as 136 people were injured, 13 of them critically. Another 18 people are missing, officials said.

The blast in the city of Coatzacoalcos in eastern Veracruz state was so powerful it was felt 10 kilometers (six miles) away. It shattered windows and forced the evacuation of schools and businesses.

Crews kept working Thursday to cool off one particularly hard-hit area of the plant, hosing it down with water.

Investigators have yet to enter this area because of fears that structures might collapse, Luis Felipe Puente, a senior civil protection official with the state interior ministry, told Milenio television.

"We have searched the affected area within the complex, and sadly so far located 13 victims," Puente wrote earlier on Twitter.

The blast at the Petroquimica Mexicana de Vinilo (PMV) plant was caused by some kind of leak, said Jose Antonio Gonzalez, chief executive of Pemex.

The plant "uses flammable materials like chlorine and ethanol but we do not know the cause of the leak," he told Televisa television.

He assured locals that the toxic cloud caused by the blast had been "dissipated by the wind" and posed no risk.

Pemex co-owns the plant with a private company, Mexichem, which operates the facility.

Some 100 plant workers and about 2,000 residents had to be evacuated, according to Veracruz governor Javier Duarte.

Duarte rushed to the scene of what he said was "a very strong explosion," where fire crews had the blaze under control.

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As the situation unfolded, he urged people living in the vicinity to remain indoors as the "cloud of chemicals" dissipated.

School classes in Coatzacoalcos, a port city, and nearby communities were suspended.

Pemex said Wednesday afternoon the situation was "completely under control."

- Agonizing wait -

Soldiers set up a security cordon several kilometers from the plant, where workers' families flocked at dawn, anxiously awaiting news of their loved ones.

"We don't know what happened to Miguel Angel. He was working for a company inside the area affected by the explosion," a relative told AFP.

Many outside the plant said they feared the number of missing was far higher than the 18 people announced by officials.

Residents were still struggling to get over the shock caused by the blast, which blew out the windows of stores and homes.

"I left my house running. I thought the whole city was going to catch fire," said Marcela Andrade Moreno.

Other terrified residents feared a repeat of the 1991 explosion at the same facility. The death toll from that incident officially stands at six, although local media insist the number is much higher.

"We live in a time bomb," said Abelardo Garduza, a merchant from the village of Allende located a few kilometers from the plant.

Pemex has had to deal with several deadly accidents at its land-based and offshore facilities in recent years.

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 killed 37 people.

Accidents have hit several oil rigs in the Gulf of Mexico, while fires have erupted in pipelines after fuel thieves punctured them.

In February, two people were killed and at least seven injured in a blaze at a Pemex oil platform off the coast of Campeche, also in the southeast.

Pemex provides one-fifth of the Mexican government's revenue but has posted huge losses amid crumbling production and oil prices.

The 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 aging infrastructure.