Greenpeace said Grupo Urbis, the private contractors hired to clean an area near the city of Ixhuatlan de Sureste, failed to achieve the aims it had set itself.

Pemex, often at loggerheads with ecologists for its environmental record, rejected Greenpeace's accusations.

It said the clean-up began in March last year and was concluded on June 22. A private company was hired to examine the work of Grupo Urbis and, supervised by inspectors from the government's Profepa ecological watchdog, produced a final evaluation on July 24.

"The results obtained complied with Profepa's internal criteria for the restoration of contaminated sites. For that reason, we must suppose that Greenpeace's samples ... were taken before the start of restoration and clean-up works," Pemex said.

Pressure groups have long criticised Pemex for damaging the environment, especially along the Gulf Coast. They say leaking oil pipes have polluted streams and rivers.

Earlier this month, Profepa published a survey of industry's environmental performance and said Pemex was the country's dirtiest firm in terms of air pollution.

Pemex says it has improved its environmental awareness in recent years and has spent millions of dollars cleaning up its mess and bringing its waste management methods up to date.

The Veracruz case has been a thorn in the side of Pemex, yielding a landmark court decision against the oil monopoly last year.

In August, arrest warrants were issued against two Pemex biologists for the dumping of toxic waste, the first ever arrest warrants carried out against the company.

Greenpeace Mexico Director Alejandro Calvillo, citing a study from the Mexican College of Civil Engineers, said on Thursday hydrocarbon levels remained thousands of times above the target pledged by Grupo Urbis and the standards of Profepa.

He said the study was conducted in November 1999, after the date Pemex said the cleanup was concluded.

Samples showed the presence of hydrocarbons in the soil ranged from 10,000 to more than 1 million parts per million in the affected area. Grupo Urbis promised to rein in the levels to less than 5,000 parts per million, Greenpeace said.