A gang of heavily armed men stormed a party inside a house in the border city of Ciudad Juárez killing 13 teenagers in the early hours of yesterday morning. A ­further 17 young people were injured in the attack, which was apparently a mistaken drugs hit.

The gunmen arrived in a convoy of up to seven 4x4s, according to local reports. While some gunmen closed off surrounding streets, others burst into the party and started shooting to kill.

There were conflicting reports as to whether the victims were celebrating victory in a local American Football championship, or had gathered to watch a boxing match. It was unclear last night as to why they were targeted, but it was immediately assumed the attack was by one of the drug trafficking gangs struggling for supremacy in the city.

"The men were well-armed. They went into the house and shot at everyone, you could hear the gunfire all around," a neighbour said.

Army spokesman Enrique Torres said the dead were from 15 to 20 years old, and an additional 17 party-goers were wounded, some critically. "They were about 15 men, they closed off the surrounding streets and began shooting at the house as they moved inside," he said.

Witnesses of the slaughter told the Associated Press they thought those behind the attack were acting on false information.

"It must have been a huge mistake," said Martha Lujan, who lives nearby.

Ciudad Juárez is just across the US border from El Paso in Texas, and is the bloodiest front in the turf wars raging around Mexico, which intensified following a government offensive launched three years ago. More than 16,000 have died since in related violence. Last year, more than 2,000 people were killed in Juárez, about a third of the national total.

The city has also had Mexico's heaviest military presence, with 10,000 soldiers patrolling the streets for most of last year. Last month the government announced it was reducing the number of soldiers present and increasing the number of federal police officers in the city. The killing has continued regardless.

Most victims in Juárez die in ones or twos, often small-time dealers associated with one gang and killed by a rival gang. But the ­arbitrariness of much of the ­violence has been underlined by a growing number of deaths with no apparent link to the ­underworld. There have also been periodic massacres, such as two attacks on drug rehabilitation clinics in September last year which left 28 dead.

Another witness to yesterday's massacre, who only gave his first name, Hector, because he feared retaliation, said he was just outside when the gunfire broke out. He said the party was an innocuous gathering of friends targeted in error. "I think there was some confusion," he said. "We're seeking justice."

The ongoing drug feuds in Juárez are said to be caused by the Sinaloa cartel trying to out-muscle the Juárez cartel at one of the most important points for smuggling drugs into the US. In the 1990s, the Juárez cartel was the most powerful trafficker, but in recent years its influence has waned. The Sinaloa cartel is led by Mexico's most infamous trafficker of the moment, Joaquin El Chapo Guzman.

In other drug violence elsewhere yesterday, 20 gunmen attacked a police station in the Pacific port city of Lázaro Cárdenas with grenades and assault rifles, killing a police officer and a mother and her son who were in the building to pay a fine.

Further north, in the city of Navolato, the bodies of three women and two men were found in their vehicle, which was riddled with bulletholes.