|Vigilante groups have been joining in the search for the students in Guerrero state|
The 43 have been missing since they clashed with police almost three weeks ago in the town of Iguala.
Vigilantes who joined the search said they had found six new burial pits, at least two of which contained what they believe are human remains.
The search had been stepped up after forensic tests showed bodies found on 4 October were not those of the students.
The latest burial pits were found by members of a group of vigilantes who had travelled to Iguala to help with the search.
|Student hangs up posters reading "They took them alive, we want them back alive"|
at the fence of the Attorney General's Office
They searched three of the remaining four and said they found what looked like human remains, clothes and hair in two of them.
If confirmed, this would bring the total number of mass graves found around Iguala since the students' disappearance to 19.
So far, forensic experts have only concluded tests on 28 bodies found on 4 October. They could not be matched with the DNA provided by the relatives of the students, raising questions as to who was in the mass grave.
It is also not clear how long ago the grave may have been dug and by whom.
About 50 people have been arrested in connection with the students' disappearance, with the vast majority being local police officers.
The officers are believed to have been working for a drugs gang, known as the Guerreros Unidos.
The missing students all attended a teacher training college in Iguala, about 200km (125 miles) south of Mexico City.
The college has a history of left-wing activism but it is not clear whether the students were targeted for their political beliefs.
They disappeared after clashes with the police on 26 September in which six people died. Eyewitnesses reported seeing them being bundled into police vans.