ACAPULCO, Mexico (AFP) - Two gang hitmen have admitted to killing 17 of 43 students missing in southern Mexico since last week, a top prosecutor said.

Inaky Blanco, chief prosecutor of Guerrero state, also said that 28 bodies were recovered from a mass grave outside the town of Iguala but that it would take at least two weeks to determine if they are part of the group of missing students.

The students disappeared after Iguala municipal police officers shot at buses that the students had seized to return to their homes on Sept 26.

Blanco reiterated on Sunday that the town's police force has links to the Guerreros Unidos drug gang. He said a gang leader known as "El Chucky" ordered two hitmen to abduct and kill the students.

The gang members made students come out of a bus, "they grabbed 17, took them to the top of a hill in Pueblo Viejo where they have clandestine graves and where they say they killed them," Blanco said.

Authorities found a mass grave in the community of Pueblo Viejo on Saturday. But Blanco said it will take 15 days to two months to confirm whether the 28 bodies found there are those of the students.

Some of the bodies were mere fragments that had been burned, he said. "At the pits found in Pueblo Viejo, a bed of branches and tree trunks was placed on which the bodies of the victims were laid and a flammable substance was used," Blanco said.