Six people have been killed in Mexico after men armed with assault rifles opened fire at a family party in the border city of Ciudad Juarez. Mexican police said on Saturday that the men entered through the back of the house, surprising the guests. Adrian Sanchez, a spokesman for the Ciudad Juarez municipal police, said the attack occurred on Friday night, near the centre of the city. Two men died from the hail of gunfire at the house, while another three men and a woman died in hospital from their wounds, he said. Ciudad Juarez, which borders the US city of El Paso, in Texas, is a battleground for drug cartels vying for shipping routes into the US. More than 6,800 people have been killed in there since early 2008. Elsewhere in Mexico, four people died in the affluent city of Monterrey during a shootout between marines and suspected drug cartel members. Mexican authorities said on Friday that one marine and three suspected drug gang members were killed in the violence. Three marines were also wounded.