The top prosecutor in Mexican state of Veracruz has confirmed that more than 250 skulls have been dug up in what appears to be a drug cartel mass burial ground on the outskirts of the city of Veracruz.

Jorge Winckler, the state prosecutor, said on Tuesday that the clandestine burial pits appear to contain the victims of drug cartels killed years ago.

"For many years, the drug cartels disappeared people and the authorities were complacent," Winckler said, in apparent reference to the administration of fugitive former Governor Javier Duarte and his predecessors.

In an interview with the Televisa network, Winckler did not specify when the skulls were found or by whom.

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On Monday, when the discovery was first reported in this southeastern state, Winckler said investigators were likely to find more remains.

But they appear to have been found over the course of months.

Victims' advocacy groups like Colectivo Solecito have excavated and pressed authorities to excavate such sites to find missing loved ones.

The skulls and other bones were found in a wooded area known as Colinas de Santa Fe, where activists have been exploring since at least mid-2016, sinking rods into the ground and withdrawing them to detect the telltale odor of decomposition.

When they find what they believe are burial pits, they alert authorities, who carry out the final excavations.

Winckler said excavations have covered only a third of the lot where the skulls were found, and more people may be buried there.

"I cannot imagine how many more people are illegally buried there," Winckler said, noting that the state has reports of about 2,400 people who are still missing.

"Veracruz is an enormous mass grave," he said.

Government 'inaction'

The victims' advocacy groups have criticised authorities for doing little to try to find or identify the state's missing people, many of whom were kidnapped and never heard from again.

Al Jazeera's John Holman, reporting from Mexico City, said that one mother of a missing person told him that her family received "very little help" from state authorities in finding her son.

"Veracruz is a real epicentre in the violence that is being felt through various areas of Mexico," he said.

Our correspondent also reported that the country's top prosecutor has yet to take action on the latest discovery, adding that federal authorities are "keen to sort of dampen down talk about the violence being suffered in the country".

Veracruz had long been dominated by the ferocious Zetas cartel. But the Jalisco New Generation cartel began moving in around 2011, sparking bloody turf battles.

Drug cartels in other parts of Mexico have deposited victims' bodies in mass graves before.

In the northern state of Durango, authorities found more than 300 bodies in a clandestine mass grave in the state capital in excavations starting in April 2011.

More than 250 bodies were discovered in April 2011 in burial pits in the town of San Fernando, in Tamaulipas state, close to the US border.

Drug gangs in some places in Mexico have taken to burning or dissolving their victims' bodies in corrosive substances in order to avoid discovery.

But the victims in Veracruz appear to have been buried relatively whole.