COATZACOALCOS, Mexico (AFP) - The death toll from a powerful explosion at a petrochemical plant in Mexico climbed to 24 on Thursday (April 21) after the authorities were able to enter the facility, state-run oil giant Pemex said.

"We have confirmed 24 deaths," up from the previous toll of 13 from Wednesday's blast, chief executive Jose Antonio Gonzalez Anaya told a news conference, adding that the number of missing dropped from 18 to eight.

Pemex co-owns the plant. Many outside the plant said they feared the real number of missing was far higher.

Anguished relatives nervously awaited news of their loved ones on Thursday outside the plant, where the explosion also sent toxic smoke billowing into the air.

Starting at dawn, family members flocked to a military cordon set up around the plant after Wednesday's explosion, which blew out the windows of nearby stores and homes and triggered panic among locals still haunted by a deadly blast at the same facility 25 years ago.

About 300 people gathered outside the plant in Coatzacoalcos, in the eastern state of Veracruz, some weeping loudly, some scuffling with soldiers.

"We want them to cut to the chase and give us (our relatives). If they don't let us through, we'll force our way in," said Mr Guadalupe Torres, whose 21-year-old brother Fernando was working at the plant and was among the missing.

A worker who survived the explosion told journalists some 300 employees were on site when the blast occurred.

"I was out back when the first explosion came. We saw the windows collapse, the iron bars crumple, because (the facilities) are extremely old," he said.

"When the second explosion happened, I saw bodies fly off the scaffolding."

The Pemex CEO said the explosion was caused by a gas leak.

The plant, known as Petroquimica Mexicana de Vinilo (PMV), "uses flammable materials like chlorine and ethanol but we do not know the cause of the leak", he told Televisa television.

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

An employee of a contractor at the site, Jose Antonio Galicia, said the plant had been registering leaks for weeks.

The release of toxic columns of black smoke sowed panic in the area as officials initially warned residents to stay indoors.

But the Mexican authorities and Pemex officials later said the cloud posed no threat to the population.

Around 2,000 evacuated residents returned home on Thursday, and life in the town was returning to normal as the dissipating cloud drifted into the distance - though schools remained closed.

President Enrique Pena Nieto sent his condolences to victims' families and announced he would visit the site.

The death toll could rise further as 136 people were injured, 13 of them critically.

Soldiers eventually began allowing small groups of family members through the security cordon to ask about their relatives' fate.

Some residents were still in shock.

"I left my house running. I thought the whole city was going to catch fire," said Mr Marcela Andrade Moreno, whose windows were blown out.

Other terrified locals 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 Mr 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, killing 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 south-east.

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 ahing infrastructure.