At least 28 bodies have been pulled from a mass grave in southern Mexico as authorities check if they were among a group of 43 students missing since a police shooting last week.

Inaky Blanco, chief prosecutor of Guerrero state, said on Sunday that 28 bodies were recovered but that it would take at least two weeks to determine if they were part of the group of missing students.

He said one person detained in the case had told investigators that 17 students were taken to the grave site and killed there. However, investigators had not confirmed the person's story, Blanco said.

Earlier, Lazaron Mazon, health secretary of Guerrero, told AFP news agency on Sunday "some charred" remains were among the bodies found on a hill outside the town of Iguala, where the students were last seen.

He confirmed the exhumation of at least 21 bodies since the unmarked grave was found on Saturday in the Pueblo Viejo hamlet, 200km south of Mexico City, adding that they have to be identified through DNA analysis.

A local resident told AFP the region is dominated by a drug gang and that he had seen municipal police officers going up the hill in recent days, before authorities discovered the mass grave.

Vidulfo Rosales, a lawyer for the families, said 35 relatives had given DNA samples so far.

Protests by students

As relatives nervously waited for news on Sunday, hundreds of fellow students from the missing group's teacher training college blocked the highway between Guerrero's capital Chilpancingo and Acapulco, voicing anger at the authorities.

The students held up signs reading "Fraudulent government that kills students."

Some parents who participated in the protest criticised Angel Aguirre, Guerrero's governor, promising "war" against the state leader, who appealed for calm after the grave was discovered.

They said they were shown pictures of bodies, but that they did not resemble their missing children.

However, if the bodies are confirmed to be those of the students, it would be one of the worst slaughters that Mexico has witnessed since the drug war intensified in 2006, leaving 80,000 people dead to date.

The grim find came a week after the students disappeared when gang-linked municipal police shot at buses they had seized to return home from Iguala, where survivors say they had conducted fundraising activities.

In all, three students were killed in the shooting and another three people died when suspected gang members shot at a football team's bus outside Iguala later that night.

Witnesses say several students, who are from a teacher training college known as a hotbed of radical protests, were whisked away in police vehicles.

Rights panel to investigate

Describing the mass grave's discovery, Juan Lopez Villanueva, an official from the National Human Rights Commission, said that six pits were found in Pueblo Viejo, up a steep hill probably inaccessible by car.

The pits outside Iguala were found after some of the 30 suspects detained in the case told authorities about their location, Blanco said.

The detainees include 22 police officers, who were arrested soon after last weekend's violence.

Blanco said on Saturday evening that eight more people had been arrested in the case, including seven members of a crime gang.

An arrest warrant has been issued for the mayor, who has fled.

The National Human Rights Commission opened its own investigation into the case for possible "serious human rights abuses", such as extrajudicial executions and forced disappearances by Iguala city police.

It said in a statement on Sunday that it had warned about the "delicate" situation in Guerrero, where poverty feeds social unrest and drug gangs clash over territory.

Source: Agencies