CIUDAD JUAREZ: At least 18 people were killed when gunmen stormed into a drug treatment center in northern Mexico's violence-plagued border city of Ciudad Juarez, security officials said.

The gunmen entered the facility and attacked 23 people, killing 18 and leaving five more wounded, a source in the public security department told AFP.

"There could be more bodies," the source said, adding that the dead were found near a small area of an entranceway to the center.

The attack capped a particularly bloody 24-hour period in Mexico that also saw the killing of 21 people overnight in drug-related violence in Chihuahua state and the murder of the number two security official in President Felipe Calderon's home state of Michoacan.

Ciudad Juarez, just across the border from El Paso, Texas, is the bloodiest stage for an ongoing turf war between Mexican drug cartels battling for the right to control the trafficking of drugs into the United States, the world's largest market for cocaine and marijuana.

Last year in a similar attack, eight people were gunned down at a Mexican clinic that treats drug addicts.

More than 9,600 people have died in the drug-related violence since 2008, despite Calderon's deployment of some 36,000 troops and police across the country to try and stem the bloodshed.

According to an AFP tally based on police data, in the first seven months of 2009 there were 1,161 murders in Ciudad Juarez alone despite thousands of police and soldiers sent there to maintain security.