Mexico has sent 80 soldiers and an army helicopter to the western Michoacán state after a shooting in which at least 13 police officers were killed.
The police were carrying out a court order in the town of El Aguaje when their convoy was ambushed by unidentified gunmen on Monday.
A powerful criminal group, the Jalisco Nueva Generación Cartel, is believed to have carried out the attack.
Mexico’s president defended his security policies following the attack.
At a news conference on Tuesday, Andrés Manuel López Obrador said his tough policies on drug crime would eventually pay dividends.
“I’m optimistic we’ll secure peace… we’re completely dedicated to this issue, but [past governments] allowed it to grow,” the president was quoted as saying by Reuters news agency.
Mexico’s federal authorities had earlier said that 14 police officers were killed, but the death toll was later revised by state officials.
Michoacán is a hotspot for violence linked to turf wars between drug cartels.
What happened in El Aguaje?
Police patrol vehicles were ambushed as they passed through the town.
Reports say the convoy was surrounded by heavily armed men in a number of pick-up trucks who then fired on the officers and set their vehicles on fire.
El Aguaje is considered to be of strategic importance between two battling cartels: the Jalisco Nueva Generación Cartel (CJNG) and a splinter group of the Knights Templar Cartel called Los Viagras.
A message left at the site suggested the attack was carried out by gunmen connected to the CJNG.
The supposed leader of the CJNG was killed by Michoacán police less than a week ago.
Michoacán has suffered cartel-related violence for years. In August, nine people were found hanging from a bridge with seven other corpses found on the road.
The federal government offered assistance to the state authorities after Monday’s attack.
Michoacán Governor Silvano Aureoles Conejo said there would be “no impunity” for the attack on his officers.
However, the Jalisco cartel has grown much more powerful in recent years and there have been no significant victories against them by either the state or federal government, the BBC’s Will Grant in Mexico City reports.
Despite the government’s efforts to tackle drug crime, last year saw a record number of murders with more than 29,000 recorded.
Worse still, this year could be set to surpass that figure.