"Self-defense" groups confronting a drug cartel in the western state of Michoacan have agreed to join government law enforcement forces after months of firefights with gang members, many times as federal police and troops stood by.

The government announced Monday that it had reached a deal with vigilante leaders to incorporate the armed civilian groups into old and largely forgotten quasi-military units called the Rural Defense Corps. Vigilante groups estimate their numbers at 20,000 men under arms.

"The self-defense forces will become institutionalized, when they are integrated into the Rural Defense Corps," the Interior Department said in a statement.

Vigilante groups began springing up almost a year ago against the Knights Templar cartel, which ruled many parts of Michoacan with an iron fist, demanding extortion payments from businesses, farmers and workers. The civilian forces have gained ground in recent months, seizing a number of towns from the gang, sometimes after intense gunbattles.

The rise in fighting proved an embarrassment for President Enrique Pena Nieto, drawing criticism that the administration brought on the rise of the armed groups by failing to stop the cartel's abuses, and the government hopes the agreement will help restore order and get it back in control.

More federal police and soldiers were dispatched to the troubled region after a particularly violent weekend early in January, but the vigilantes have refused to lay down their guns and return home until all of the cartel's major leaders are arrested. Officials announced that one of the cartel's top four leaders was captured early Monday.

Even before the new agreement, police and soldiers already largely tolerated, and in some cases even worked with, the vigilantes, many of whom are armed with assault rifles that civilians are not allowed to carry.

Vigilante leaders will have to submit a list of their members to the Defense Department, and the army will apparently oversee the groups, which the government said "will be temporary." They will be allowed to keep their weapons as long as they register them with the army.

The military will give the groups "all the means necessary for communications, operations and movement," according to the agreement.

Vigilante leaders, who include farmers, ranchers and some professionals, met to discuss the agreement, but it was not yet clear for them how it would work. It wasn't known if the army would offer anyone salaries....