A massive explosion rocked a major petrochemical facility of Mexican national oil company Pemex in the Gulf state of Veracruz on Wednesday, killing at least three people, injuring dozens more, and pumping a cloud of noxious chemicals into the sky.

Luis Felipe Puente, head of federal emergency services, told Reuters that three people had died in the blast. Pemex confirmed that three of its workers had died, and said another 136 were injured, of which 88 were still in the hospital.

The firm said the explosion, which sent a huge, dark plume of smoke billowing upwards, occurred just after 3 p.m. (2000 GMT) at the facility's chlorinate 3 plant near the port of Coatzacoalcos, one of the company's top oil export hubs.

Local emergency officials said hundreds of people had been evacuated from the site. Television footage showed an initial burst of flames followed by a tower of thick smoke. A company official said local oil exports were unaffected.

What caused the blast was unclear, but Pemex initially warned local residents to keep away from the site due to what it described as a dissipating cloud of toxic fumes. TV footage showed rain clouds gathering above the plant as evening fell.

"The current situation at the plant... is under control and there is no risk to the population," Pemex said later in the evening on its official Twitter account.

Pemex Chief Executive Jose Antonio Gonzalez traveled to Coatzacoalcos late on Wednesday to oversee the response.

Petroquimica Mexicana de Vinilo, or PMV, a vinyl petrochemical plant that is a joint venture between Pemex's petrochemical unit and Mexican plastic pipe maker Mexichem (MEXCHEM.MX) was the facility hit by the blast.

Operated by Mexichem, the plant lies within Pemex's larger Pajaritos petrochemical complex. Mexichem said in a statement the explosion occurred in an ethylene unit at the plant. The company could not be immediately reached for further comment.

In February, a fire killed a worker at the PMV plant, which makes vinyl chloride monomer, also known as chloroethene, an industrial chemical used to produce plastic piping.

The incident occurred just weeks after three workers were killed and seven injured when a fire broke out on a Pemex oil-processing platform in the Gulf of Mexico.

It also came as Pemex implements deep cost cuts to cope with the rout in oil prices, and seeks to stem a slide in output. Mexico is in the midst of a historic push to lure private investors to revive its oil industry.

Pemex, which enjoyed a decades-long monopoly over Mexico's oil and gas industry until an energy reform opened up the sector in 2014, has experienced a series of high-profile accidents.

In 2013, at least 37 people were killed by a blast at its Mexico City headquarters, and 26 people died in a fire at a Pemex natural gas facility in northern Mexico in September 2012.

A 2015 fire at its Abkatun Permanente platform in the oil-rich Bay of Campeche affected oil output and cost the company up to $780 million.

Pemex said last year it had reduced its annual accident rate in 2014 by more than 33 percent. But a Reuters investigation found that Pemex was reducing its accident rate by including hours worked by office staff in its calculations.(Reporting by David Alire Garcia, Gabriel Stargardter, Liz Diaz, Dave Graham and Anna Yukhananov; Editing by Andrew Hay and Richard Pullin)