At least eight inmates have died at a prison in the city of Porto Velho in north-western Brazil, according to BBC News.

The prisoners died from smoke inhalation after fellow inmates set mattresses alight in the Enio dos Santos jail in Rondonia state.

The prison director said the clash was set off by the transfer of the leader of a criminal gang to the jail, which is controlled by a rival gang.

Officials said there could be a link to a similar incident in Roraima state.

There, 10 inmates died in clashes on Sunday between a gang calling itself First Command of the Capital, or PCC, and their rivals, Comando Vermelho (Red Command), the state authorities said.

Police initially said 25 inmates had been killed, but the figure was later revised down.

The incident in Roraima happened during visiting hours. About 100 visitors were taken hostage and later freed.

Police said they had regained control of both jails.

Brazil has the world's fourth largest prison population, with more than 620,000 inmates.

The PCC is a powerful criminal gang first established in a Sao Paulo prison in 1993.

It has since expanded to become a powerful organisation which engages in drug trafficking far beyond the prison walls.

Its members frequently clash with those of the Comando Vermelho, a gang originally founded in Rio de Janeiro.

Many of the most powerful gang leaders, with reputations for violence and brutality, are often moved from jails far away from their home states but they are still able to run their criminal network from inside prison, says the BBC's Wyre Davis in Rio de Janeiro.