CIUDAD JUAREZ, Mexico - A spasm of bloodletting by drug gangs in Mexico claimed 40 lives over a long weekend in Mexico, including at least 18 young people gunned down at a high school party, officials said.

The worst act of slaughter happened in Ciudad Juarez, Mexico's murder capital, when gunmen drove up to the house in several cars, first shooting at people gathered outside, then chasing and cornering some of the partygoers who jumped over a fence frantically running for their lives.

Most of the victims were "youngsters", said the Chihuahua state attorney general's office.

Enrique Torres, a military spokesman, said the gunmen had apparently been looking for someone who was not even at the party, which was to celebrate a birthday and a local soccer team championship.

Almost at the same time but in Torreon, in the northern state of Coahuila, 10 people were killed when gunmen rolled up in a trio of Hummers and opened fire on a crowd inside a bar.

Early Monday gunmen again opened fire in Ciudad Juarez at a bar, killing five people and injuring six others, police said.

Meanwhile a clash between two organized crime groups in Madgalena, Sonora state, left seven people dead early Monday.

The Juarez cartel led by Vicente Carrillo and the Sinaloa cartel led by Joaquin 'Chapo' Guzman are battling for control in Chihuahua state, trying to mark their territory, authorities say, with no regard for human life.

The war waged by rival drug cartels in Mexico has already left more than 15,000 people dead. The government has deployed 50,000 troops and thousands of police in an effort to rein in the violence.