Four prison officials in northern Mexico have been arrested for allegedly allowing several inmates to carry out three separate contract killings since February, Mexico's federal prosecutors' office said.

Ricardo Najera, a spokesman from the attorney general's office, said on Sunday that the prisoners were set free from the Gomez Palacios Social Rehabilitation Centre in the northern state of Durango to carry out the killings.

He said the inmates were given weapons that were traced to the prison through bullets found at crime scenes and matched to weapons belonging to four officials.

Margarita Gomez, a prison warden, and three other prison officials have been accused of authorising the prisoner-for-hire scheme and have been placed under a form of house arrest pending further investigation.

"These criminals carried out their executions as part of the settling of accounts between rival gangs and disgracefully these cowardly criminals then murdered innocent civilians on their way back to their cells," Najera said.

Dozens of people are believed to have been killed in three massacres in Torreon, an industrial city in northern Durango state close to Gomez Palacios, where the prison is located.

The deaths include 17 people shot dead at a party on July 18; 15 people killed inside a bar on May 18, and 10 people killed on February 1 in the same city.

The slayings are part of the relentless bloodletting among rival drug cartels that has been blamed for about 25,000 deaths since December 2006, when Felipe Calderon, the president of Mexico, launched a massive crackdown against drug crime and deployed 50,000 troops across the country.

The worsening violence has alarmed the United States and risks undermining investor confidence in Mexico, where the industrial sector depends heavily on export sales and foreign investment.