* Pemex says death toll from blast could still rise
* Pemex CEO says unclear what caused the accident
* Remediation of the site could take up to a year
* Blast also injured 136 people, 13 of them seriously (Updates number of deaths, comments from press conference, president's visit)
By Lizbeth Diaz
COATZACOALCOS, Mexico, April 21 (Reuters) - Twenty-four people died after a leak caused a deadly petrochemical plant blast, and the death toll could still rise, Mexican oil giant Pemex said on Thursday, the latest in a series of fatal accidents to batter the company.
Pemex CEO Jose Antonio Gonzalez Anaya, who traveled to the site of Wednesday's blast near the port of Coatzacoalcos, one of Pemex's top oil export hubs, told local television it was unclear what caused the accident.
The massive explosion at the facility's chlorinate 3 plant in the Gulf state of Veracruz also injured 136 people, 13 of them seriously. Another 18 people were unaccounted for, and one badly damaged part of the plant had yet to be scoured.
"We know there was a leak, what we don't know is why, but everything points to an accident," Gonzalez Anaya said.
He shared an updated death toll at a press conference late on Thursday, adding that remediation of the site could take up to a year. He denied the blast was tied to the economic problems of Pemex, which is trying to stem sliding output and slash costs as it creaks under the pressure of low crude prices.
The sharp odor of ammonia filled the air and the plants' turbines still streamed gray smoke on Thursday afternoon, where local and municipal police, as well as marines, blocked the entrance to the facility.
Most officials wore blue face masks to protect against the fumes, while family members crowded around, their faces uncovered, demanding more information on missing relatives and at times throwing objects at the officials or pushing them. Others held hands and prayed for the missing and dead.
"We are desperate because no-one is coming out to show their face," said Ancelma Cordero, 49, whose 21-year-old brother is one of the missing and has not responded to his cellphone.
She said she had been waiting since the prior night and her head was starting to hurt.
"They told us we were breathing toxins and we should leave," she said of authorities. "But ... if we leave, they could make the bodies disappear."
Calling it a "tragic accident", President Enrique Pena Nieto headed to the region late Thursday to tour the facility with local officials and speak with victims and their families.
The blast occurred at a vinyl petrochemical plant that is a joint venture between Pemex's petrochemical unit and majority owner Mexican plastic pipe maker Mexichem. Pemex operates the larger petrochemical complex where the plant was located, known as Pajaritos.
The plant produces some 900 tons a day of vinyl chloride monomer, also known as chloroethene, an industrial chemical used to produce plastic piping. The joint venture had forecast sales of $260 million this year.
Shares in Mexichem closed 5.2 percent lower on Thursday.
"This is neither the time for excuses nor finding those to blame," Juan Pablo del Valle, Mexichem's chairman, said on Twitter. "It is the time to tend to the injured, be accountable and support all those affected."
In February, a fire killed a worker at the same plant, the latest in a litany of safety disasters that have plagued the state oil giant.
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. (Reporting by Tomas Bravo, Anahi Rama, Veronica Gomez, Gabriel Stargardter and Joanna Zuckerman Bernstein, writing by Alexandra Alper and Anna Yukhananov; Editing by Simon Gardner, Alan Crosby and Himani Sarkar)