Gunmen have killed 13 people in the southern Oaxaca state, police say.

Officials say hooded men travelling in several cars stormed a ranch killing the owner and his three sons as well as four other people.

The Mexican authorities say the gunmen then killed five local police officers who tried to fight back.

The incident follows the deaths of 19 people in Tamaulipas state over the past three days in confrontations between police and illegal groups.

More than 15,000 people are believed to have been killed in drug-related incidents in Mexico in the last two years.

Local official Netolin Chavez said that investigators believe the latest attack, in the town of San Vicente Camalote, could be linked to the drugs trade.

State police have been sent to patrol the town.

The Pacific coast state of Oaxaca is considered to be a drug trafficking corridor, although violence has not risen to the levels seen in Mexico's northern border region.

In that region Ciudad Juarez, by the border with the US, has seen the most deaths. More than 2,600 murders were recorded in Ciudad Juarez during 2009.