At least eight people were killed in clashes in southern Mexico over the weekend when police and members of a teachers' union faced off in violent confrontations.
Violence erupted on Sunday when police dislodged protesters blocking a highway in the southern state of Oaxaca, a hotbed of dissent from radical teachers' groups opposed to education reforms pushed through by the government three years ago.
Speaking on local radio early on Monday, Jorge Ruiz, Oaxaca's state secretary for public safety, said eight people died in two separate confrontations, raising the death toll in the clashes from a previous tally of six.
He said six people died near the town of Nochixtlan, about 80 kilometres northwest of the city of Oaxaca, while two others were killed in related protests in Juchitan, in the southeast of Oaxaca state.
Mexican President Enrique Pena Nieto said on Twitter he was sorry for the loss of life and the federal government would help Oaxaca state investigate the incident.
"I've given instructions so that ... actions can be taken to solve the conflict," he wrote.
The violence is the latest in a series of setbacks to Pena Nieto's government, which has faced widespread criticism for its failures to crack down on graft and impunity, contain drug gang violence or jumpstart the economy.
The violence came as Christof Heyns, the United Nations' Special Rapporteur on extrajudicial, summary or arbitrary executions, said on Monday that police accountability in Mexico was insufficient.
The unrest has escalated since police arrested the leader of the local teachers' union earlier this month.
Ruben Nunez, head of one of the most combative factions of Mexico's CNTE union, Oaxaca's Section 22, was detained on suspicion of money laundering.
Miguel Zurita, a CNTE representative in Oaxaca, said that when police arrived to dislodge Sunday's protest near Nochixtlan, they were unwilling to enter into a dialogue.
"What we lived through yesterday was something brutal, something that has no name," he said. "They arrived armed and they arrived shooting."
The Mexican government, however, defended its handling of the protests.