Authorities have uncovered mass graves on the outskirts of a southern Mexican town where 43 students disappeared after a deadly police shooting.

"Pits with bone remains" were found outside the town of Iguala, about 200 kilometres south of Mexico City, Guerrero attorney-general Inaky Blanco told reporters.

Mr Blanco said the remains would be sent to Mexico's forensic service to determine whether or not the corpses were those of the missing students.

He declined to say how many graves or corpses had been found.

The 43 students disappeared last weekend after Iguala municipal officers shot at buses that the group had seized to return to their teacher training college near the state capital Chilpancingo.

Three students were killed. Another three people died when police and suspected gang members shot at another bus carrying football players on the outskirts of town.

Witnesses told AFP news agency that the officers took away several students in patrol cars after the shooting. One witness said he saw 30 to 40 people taken away.

The officers are suspected of having links to criminal gangs, raising fears about the students' fate in a country where drug cartels often bury their victims in mass graves.

A police official said the pits on the outskirts of Iguala were found thanks to an anonymous phone tip-off.

"We are very worried. The families are very anxious," said Vidulfo Rosales, a human rights lawyer representing relatives of the missing.

Authorities detained 22 Iguala officers over the shootings and issued arrest warrants for the town's mayor and security chief, both of whom have disappeared.

The United Nations has urged Mexican authorities to conduct an "effective and diligent" search for the missing, calling the case "one of the most terrible events of recent times".

Guerrero is one of the most violent states in Mexico, with gangs known as "Guerreros Unidos" and "Los Rojos" engaged in bloody turf wars.

AFP/Reuters