By Ana Isabel Martinez

MEXICO CITY, March 8 (Reuters) - Mexico's state-run Pemex is burning natural gas from its Ixachi field at a rate of "practically zero," according to the firm's chief executive, adding it missed a deadline to end the environmentally harmful practice due to tests at a processing plant.

Pemex promised late last year to stop flaring, or the burning off of natural gas, from Ixachi by mid-January, amid mounting pressure to improve its poor environmental record.

But satellite data that scientists analyzed exclusively for Reuters - as well as a visit by reporters to the site - showed it not only continued, but also increased.

When asked about the missed January deadline at the field in southeast Veracruz state, CEO Octavio Romero told Reuters late on Tuesday that since the start of the year Pemex has made infrastructure adjustments.

"We're already at a point where the flaring is at practically zero," he said, adding that some 170 million cubic feet of gas originating at Ixachi is processed at its nearby Papan plant.

Romero did not specify how much gas continues to be burnt off from Ixachi or exactly when flaring levels were reduced.

Pemex achieved lowered flaring after completing delayed mechanical and personnel tests at Papan, Romero said.

But the figure cited by the Pemex chief, a close confidant of President Andres Manuel Lopez Obrador, is about half of what Pemex previously said the plant is meant to process.

Mexico is one of the world's top 10 offenders for gas flaring.

Romero emphasized that under his watch the company is investing in infrastructure to minimize the practice - following what he described as years of neglect from his predecessors.

"For us, the environmental issue is a priority."

Gas is sometimes flared for security reasons if a plant determines it cannot safely process the gas.

But the environmental toll is increasingly clear.

In a bid to address it, the World Bank's Zero Routine Flaring initiative aims to commit oil companies to end routine gas flaring by 2030. (Reporting by Ana Isabel Martinez; writing by Stefanie Eschenbacher; Editing by David Alire Garcia and Josie Kao)